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/■ i«* iViJI

[From The American Geologist, March, 1891.]

PETROGRAPHICAL DIFFERENTIATION OF CER-
TAIN DYKES OF THE RAINY LAKE REGIOW.

By Andrew C. Lawson, with analyses by F. T. Shutt, M. A., F. C. I.

Conininnicatloii No. 2.

At the Toronto meeting of the A. A. A. S. the writers sub-
mitted a paper in abstract bearing the above title. The material
for the full paper was not at the time of the meeting complete,

•».

*..; ■ i

154

The American Geologist.

March, 1891

and various vicissitudes have since then interfered with the work.
However, as some of the facts observed are of interest it is pro-
posed to give them here without attempting to elaborate them so
fully as was originally contemplated.

The dykes of the Rainy Lake region shov,' in a marked degree
that variation from a fine texture at the dyke walls to a coai'se
texture in the middle of tbe dyke, which is more or less charac-
teristic of dykes the world over. In the abstract referred to it
was stated that " On examination of the dykes in question, it be-
came apparent that this variation in the physical appearance of
the dykes is not simply one of texture or degree of coarseness of
the constituent minerals, but it is rather the incidental concomi-
tant of important structural, tMneralogical and chemical variations
which appear verj' constantly in the same way in different dykes."
It will be the object of the present paper to establish this general
statement by giving some account of the facts upon which it is
based.

A dyke about 150 feet wide which traverses Stop island on
the south side of Kainy lake, in which the variations alluded to
are strongly accentuated, will be first desci'ibed somewhat in de-
tail. From this dyke a series of four specimens was taken, viz. :

I at the contact with the dyke wall.

II at four feet from the contact.

III at fifteen feet from the contact.

IV at seventy-five fe'>^ from the contact (middle of dyke).
Textural variation. — To .. unaided eye there is apparent a

very distinct gradation in texture from that of an aphanitic rock
at the contact to that of a coarse gabbro or diorite in the middle
of the dyke. The gradation is rapid in the first four feet, less ao
from four feet to fifteen, and scarcely perceptible from fifteen feet
to the middle of the dyke ; the rock on which the observations
were made in the field presenting a continuous, clean, fresh, glac-
iated surface. In order to arrive at some definite information
regarding the gradation in texture, careful measurements were
made of the constituent minerals in thin sections of the different
specimens. The following are the results of these measurements :

I Ground mass. ^

Pyroxene — .argest grains 0315 mm

Ave. diameter 030 mm

Feldspar in slender needles.

Ave. size 004 X .053 mm

*. ; * ' •' : •• • • :
•• •'♦ •/. .•!.••: : *
: ;;;•.. ••

•. •.•.••/.•**..•'••::'*:*'••/.*•.

••• .••

Petrograpkical Differentiation of certain dykes. 155

Magnetite, ave. diameter 0147 mm

Porpliyritic crystals.

Pyroxene, largest polysomatlc grain 1. 1:,M) mm

Feldspar, largest lath-shaped crystal ()")<) X .. ')()() mm

II Pyroxene, approx. ave. diameter 840 mm

Feldspar, " " size noo X .or»t) mm

Magnetite, " " diameter 12(5 mm

III Pyroxene in polysomatlc masses, ave. diameter. . .. ;.'.000 mm

Feldspar, lath-shaped crystals, largest 350 X .100 ram

Magnetite, few scattered grains, ave. diam 700 mm

Quartz, ave. diameter 050 mm

IV Pyroxene, much altered to hornblende, larger gr. . 1.000 mm

Feldspar, ave. size of larger grains 3.000 mm

Magnetite, few large grains, diam 700 mm

Quartz, larger grains 840 mm

Structural variation. — The specimen taken at the conta(!t with
the dyke walls (I) appears in section as a porphyrite. The
ground mass is a fine ophitic felt work of plagioclase and greenish
yellow pyroxene, with viridite thickly studded with granules of
magnetite, all of the latter belonging probably to the final con-
solidation of the magma. The porphyritic constituents are in the
order of their generation (1) Plagioclase in lath-shaped crystals,

-■^"---■^^-^--'■■--'^•""-'"^--''^""-^-'- Fio. 1.-"- -—'- '--^^■■--—- ■

Stop Island dyke. — Section of dyke-rock at contact with dyko wall.
X38. a. polysomatlc augite; f. plagioclase. Illustrating porphyrite
structure.

.56

The American Geologist.

March, 1801

either in distinctly isolated individuals or aggregated together in
irregularly radiating clusters ; (2) Augite, in rounded or bleb-like,
colorless poly somatic masses. The plagioclase is occasionally in-
cluded in the augite. The microscopic aspect of this portion of
the dyke is shown in fig. 1 .

In the specimen taken only four feet from the last, i. e. , four
feet from the dyke wall, the structure of the rock presents a
marked contrast to that just described. There is no ground
mass, and the section presents the character of a typical ophitic
diabase as shown in fig. 2.

<^^f^^^^f^

Fig. 2.

Stop Island dyke. — Section of dyke-rock four feet from dyke wall.
X38. a. Augite; f. plagioclase; h. hornblende; m. magnetite. Illus-
trating ophitic or diabase structure.

In this portion of the dyke the dominant minerals are polj^so-
matic augite and green hornblende. The latter mineral is in part
derived from the augite, while part of it presents no evidence of
such derivation and maj' be original. These two minerals occur
in large interlocking masses in which are embedded beautifully
sharp idiomorphic ciystals of plagioclase and irregulnr grains of
magnetite. It seems probable fi-om a careful inspection of the

;

Petrograpliical Differentiation of certain dykes. 167

reliitive attiiude of the constituents that a portion «)f the aujiite is
of earlier generation than the rest, and earlier than such portion of
the hornblende as may be original. The rounded contours of the
fresher masses of pol^'somatic augite suggest analogy with the
rounded porphyritic masses shown in fig. 1. In addition to the
idiomorphic plagioclase there is a subordinate proportion of plag-
ioclase which shows no crystallographie boundaries. Occasionally
V iraall grain of quartz may be detected. At fifteen feet from the
dyke walls the structure is still ophitic. Augite is the dominant
mineral and appears to be of two generations, (1) large, irregu-
larly bounded polysomatic masses and idiomorphic crystals,
(2) allotriomorphic, interstitial between the idiomorphic plagio-
clase. Quartz is sparingl}- present, and magnetite is in large,
irregularl}' scattered grains, some of it allotriomorphicall}' devel-
oped about the idiomorphic augite.

In the middle of the dyke the structure is entirel}' different
from cither that of a porphyrite or of an ophitic diabase. It is
the tj'pe of structure characteristic of granite, gal)bro, or diorite.
All the important constituent minerals interfere with one another,
and the only idiomorphic crystals are those of accessory minerals
such as apatite. The aspect of a section of this part of the dyke
is shown in tlic drawing, fig. 3. Quartz is abundant, and the
augite appears to be entirely replaced by horn'olendo, so that the
rock would be classed with the (piartz-gabbros or (juartz-diorites
according as the hornblende is secondary or original. Considereil
simply as a hand specimen it is best termed, probably, a uralitic
quartz-gabbro.

MineraJogical variation. — The most important mineralogical
variation observable in the series of speciniens taken across the
d3"ke is the passage from a quartzless rock at the dyke wall to a
quart^ose one in the middle of the dyke. No quartz can be de-
tected at the side of the dyke. At four feet from the side quartz
may be observed in occasional grains, forming an exciHHlingly
small proportion of the constituents ; at fifteen feet it is somewhat
more al)undant, and in the middle of the dyke (juartz is a promi-
nent constituent of the rock. Another important change in the
mineralogical composition of the rock is the encroachment of
hornblende upon the augite as one passes from the dyke walls,
and the final complete replacement of the augite in the middle of

158

The American Geoloylf<t.

March, 1891

the dyke. This change, even t.hon<;h it he fine in great part to
pju-amorphiHin, testifies to an important variation in the cliaraeter
of the rock developed from tlie same magma at various distances
from tiie dyice walls. The angite nearer the middle of the dyke
would appear to be much more susceptible of paramorphic change
than that near the dyke walls. The middle part of the dyke is
also richer in accessory minerals such as apatite, biotite, epidote,
and leu(!oxene than the lateral parts.

Chemical variation. — ^he chemical variations observed in this
dyke will be gathered from a consideration of the following table
of analyses of the specimens above referred to :

Stop Island dyke.

II.

III.

IV.

SiO,

47.83

1 47.08

47.84

57.. 50

Fo,().,fFeO

4.57

0.72

5.07

AI,0,

3().r28

25.40

23.44

CaO

(i.7;.'

8.44

5.02

MixO

4.;5rJ

.5.25

2.70

KjO

trace

.(50

.45

Na,()

1.30

2.55

i 2.01

>%<>«

2.H)

.94

2.02

l-usson ig.

2.05

2.53

2.25

{)<i.2()

S.OfiO

100.27

101.12

Sp. g.

3.028

3.080

2.850

These analyses show a remarkable increase in the propor
tion of silica in the middle of the dyke over that in the lateral
parts. The difference in silica content of about 10 per cent is
sufficient to separate the specimens into two distinct rock species
according to current methods of classification. The difference is
in keeping with the quartzose character of the middle of the dyke
as compared with the quartzless character at the side, and also
harmonizes with the difference in specific gravities given in the
table.

Thus in half the space of a sharpl}' defined dj'ke onl}' 150
feet wide our study reveals variations in all of those characters
which we make use of in the description and classification of rocks.
Totally distinct types of texture, structure and composition belong
to tlie same geological unit mass. This fact suggests an interest-
ing commentary upon our system of rock classification. Is such
classification in cases like the present, or even generally, anything

Petrographival Differentiation of certain dykes. 151)

more thiin n cliisHifk-sition of hand specuneiis ? Of wluit philo-
sophic or geological value is a .claMsificatioii of specimens into
different species and types when they may all be one and the same
rock crystallized from the same magma within a few feet of one
another. The geologist who knows his rocks in the field as well
ap in the laboratory finds such classifications ver}' little expressive
of geological truth. But the differentiation in character of this
<lyke rock suggests other matters tlian a criticism of classification,
namely, a consideration of the conditions under which such difi'er-
«ntiation was developed from a common magma. From the nature
of the case we are able to form fairh satisfactory conceptions as
to two conditions which are commonly regarded as having a para-

Pio. 3.

Stop Island dyke. — Section of dyke-rock seventy-five feet from dyke
wall (middle of dyke). X38. f. plagioclase; h. hornblende; q. quartz;
m. magnetite. Illustrating allotriomorpliie-granular or granitic
structure. j.

mount influence upon the solidification of rock from magma.
These are (1) the relative pressure, and (2) the relative rate of
oooling under which the different parts of the magma solidified.
The magma being confined between parallel sharply-cut fissure
-walls may fairly be assumed to have been under the same con-
stant hydrostatic pressure in an}' given horizontal plane during the

160

77*d American Geoloyist.

March, 1801

time of its solidification. It may also be fairly assumorl that
those portions of the maj^inii adjacent to the fissure walls cooled
more rapidly and solidified earlier than did the middle portion.
Thus although wc have in current petrographical literature numer-
ous references to great pressure as one of the chief causes of the
development of the coarser texture and granitic structure of the
plutonic rocks as compared with that manifested by rocks which
soli<lif3' at or near the surface, the present case seems to establish
the fact that both types of rock structure may be developed under
one and the same pressure. Difference in pressure under which
magmas solidify is, therefore, probably, not so important a cause
of the difference in structure; and texture of rocks as is generally
supposed. On the other hand we have in the case imder con-
sideration strong presumptive evidence that the rate of cooling
which must have been rapid at the sides and slow in the middle,
exercised the controlling influence over the character of the rock
developed from the magma in any given part. With regard to the
conditions which determined the chemical and mineralogical differ-
entiation of the dyke rock very little can be definitely affirmed.
It seems probable, however, that the explanation lies in the earlier
separation of the more basic minerals accompanied b}' a trans-
ference of more acid residues (or solvents) to the middle portions,
which transference was facilitated by the gradual solidification of
the magma from the dyke walls toward the middle, and by the
movement of the water constituent of the magma towards the
middle. The water of the msigma, so long as the latter remained
li(iuid, would have a tendency to escape to the surface. This
tendency, taken with the tendency of the higher portions of the
dyke to solidify more rapidly than the deeper-seated portions
would create a current obliquelj- through the magma, upward and
inward from both sides. This current would aid in the transfer-
ence from the sides to the middle of the more acid portions of
the magmn from which the more basic had separated out.

Numerous other dykes have been examined with the same gen-
eral result as that arrived at by a studj' of Stop Island dyke. In
none of these, however, was the differentiation in character found
to De quite so strongly accentuated as in the Stop Island dyke.
A series of specimens taken across the dyke which cuts the south-
east shore of White-fish bay, and which is referred to in former

PetTographlml Dlferentiation of certain dykes. 161

notes as tlu, White-fish Bay dyke, was analyzed witli the foUowhiff
results : *

Wliito-lislj I{ay dyke.

Fe^Og-fFeO

CaO
MgO

Na^O

Loss on Ig

Sp. g.

I near contact with dyke wall.

II six feet from contact.
Til thirty feet from contact.

IV sixty feet from contact (middle of dyke).
In this dyke the gradation in texture is as pronounced as in the
Stop Island dyke but the differentiation of structure is not so
marked. In I the ground mass has the character of a fine-drained
ophitic diabase and the porphyritic constituents present no -reat
contrast in size to those of later generation which have crys-
tallized around them ; and in IV the ophitic structure is not en-
tirely replaced by the granular. This dyke is noteworthy for the
abundance of hypersthene which is present near the dyke walls
This hypersthene is a porphyritic constituent, and has well defined
crystallographic form. It has not been observed in specimens
from other portions of the dyke and its occurrence recalls the
similar occurrence of enstatite in the Jack-fish Lake dyke and in
the Rat-root Bay dyke which hps been noted in a former paper
There is as in the Stop Island dyke a regular increase in tiie pro-
portion of quartz in passing from the dyke walls to the middle
and in the latter part of the dyke the augite is entirely replaced
by hornblende. The analyses of this dyke rock and of the Stop
Island dyke rock show throughout an unusually high percentao-e
of alumina. "

A dyke sixty-five feet wide cutting biotite gneiss with a
northwest strike on the north shore of Shoe Bay, Rainy Lake,

162

TIte American Geologist.

Marcli, 1891

afforded three specimens taken in the same seqnence as before.
The first, from the contact, shows a pronounced porphyrite
structure cor listing of a fine ground mass of phigioclase, augite
iind magnetite in which are imbedded lath-shap'^d crystals of rather
cloudy plagioclase of un earlier generation and polysomatic aggre-
gates of pyroxene, which in many cases is partially altered to a
serpentinous, greenish-yellov/ substance either on the periphery,
having a fresh core, or in patches and shreds through the section.
In the second specimen, taken at six feet from the contact, the
structure is ophitic and in marked contrast to that of the last.
The plagioclase is fresh, the augite is in scattered gi*ains and in
polysomatic aggregates and is more or lesL' altered to hornblende.
Quartz is present and is intergrown with the feldspar after the
liianner of pegmatite. Magnetite occurs in skeletal forms and
apatite in slender needles. In the specimens taken from tlie mid-
dle of the dyke the general structure is granular rather than
ophitic although the latter structure is observable. Augite is seen
in occasional large pol3'somatic grains with a good deal of filmy
or shreddy perimorphic hornblende and some chlorite. Hornblende
also occurs in independent masses. Quartz is ver}' abundant and
is nearly all intergrown with feldspar in pegmatitic structure.
Magnetite and apatite are present, the former in irregular scat-
tered grains and the latte'- in slender needles. A partial chemical
examination of the sptcimens from this dyke gave the following
figures for the percentage of silica and the specific gravity :

Sp.g.

Contact.

49.36
3.077

Middle.

51.04
3.007

Near the mouth of S\oe Bay, on the north side is another dyke
similar to the last. It is about seventy feet wide. Thin sec-
tions of three specimens taken from the same parts of the d3'ke
as before, present the same general features as in the dyke last
described. The rock at the contact is a porphyrite with the
usual plagioclase crystals and augite aggregates imbedded in a fine-
grained base. The latter is remarkable for the uniformly even
distribution of the magnetite grains. The porphyritie augite has
no crystal lographic boundaries and its alteration is for the most

Petrographlcal Differentiation of certain dyhes. 1C3

part marfjinr.l. No quartz was observed. At six feet the s^^ruot-
ure is again ophitic. Quartz is present. IMagnotite is in large
«ized, sparsely scattered grnins. A little pyrite also occurs. The
structure of the middle part of the dyke is for the most part
•granular hut with some idiomorphic plagioclase. The augite is
generally fresh, but has associated with it hornblende and chlorite
as alteiiition products. Quartz is abundant with uniform orienta-
tion over wide, and in the section isolated, areas. The following
are the figures for the silica percentage and specific gravity of the
three specimens :

Contact. Six feet from Middle,
coiiiact.

SiOj

48.85 47.93 49.28
3.088 3.079 i 3.016

Similar specimens were taken from a dyke on Risky Island,
Bamy Lake, which is nearly in a line with the Stop Island dyke and
the Shoe Baj^ dyke last described. The contact rock is as before
distinctly a porphyrite similar to that represented in tig. I. The
minerals are all fresh. Plagioclase in large crystals and augite in
single and in polysomatic gnuns, are imbedded in the usual fine
ground mass, which in this, and in all the dykes described, is ap-
parently holocrystalline and micro-ophitic. At six feet fr^m the
dyke wall the structure is ophitic but with some allotriornorphic
plagioclase. The augite is mostly replaced by hornblende. Quartz
is plentiful in pegmatitic develoi)ment. Magnetite or titanic iron
is abundant but without any trace of leucoxene. In the middle
of the dyke the ophitic structure is still ob.sorvable but most of
the constituent minerals ai-e allotriomorphic. The augite is very
largely replaced by hornblende. Quartz is observed to present
the same ophitic relations to the idiomorphic plagioclase as does
the {iugite, and it contains inclusions of apatite and microliter.
Leucoxene with cleavage traces of titanic iron is abundant and in
large grains. The following are the figures for the percentage of
silica and specific gravity of these specimens: ^^

Contact.

49.77
3 100

Six feet from
contact.

49.64
3.044

Middle.

siOj

Sp. g.

.52.31
3.016

164

The American Geologist.

March, 1891

On tlie south side of Rain}' river opposite Sec. 20, Tp. 5 S. ,.
R. XXVIII of the Canadian township survey, a dyke was ob-
served having a width of from 150 to 200 feet and cutting horn-
blende schists with a north-northwest strilce. No specimen was here-
obtained at tlie immediate contact, and in one a little removed
from the contact the porphyritic structure was onl}' represented by-
blebs of polysomatic augite imbedded in an ophitic base whichc
approached in texture that of the specimens taken at four or six
feet from the contact in other dykes. The ophitic structure pre-
vailed in two other specimens, one taken at six feet from the con-
tact and one from the middle. Quartz was observed in both of
these but not in the first. The percentage of silica and specifie
gravity of the first and third specimens is as follows :

Near
Contast.

Middle.

Sp. g.

49.82
3.221

50.10
3.068

Scries of specimens from several other dykes were also ex-
amined, but the limit of space will not permit of further detailed
descriptions. Generally, however, it may be said that the por-
phyrite structure almost invariably characterizes the dyke rock at.
the contact and that this rapidly grades into an ophitic structure
which in turn appears to grade very gradually into the granular
structure. The latter, it must be said, is developed to the entire
exclusion of the ophitic structure only in a few of the cases ob-
served. The increasing proportion of quartz toward the middle
of the dykes is a very constant character. In one dj'ke, namely
that on the south side of Rainy river opposite the town of Fort,
Frances, well defined crystals of enstatite were observed in the rock
at the contact as a porphyritic constituent while none of thia
mineral was observed in other parts of the dyke.

[From the Proceedings of the American Association for the Advancement

OF Science, Vol. xxxviii.]

Pbtrogiiaphical difficukntiation of certain dykes of the Rainy
Lake region. By Dr. A. C. Lawson and F. T- Shutt, M. A., F. C. I.,

Ottawa. Out.

[ABSTRACT.]

Onc of the writers has described in a former paper certain diabase
dykes of the Riv'.ny Lalie region. The present paper is the result of a more
critical investigation of the same dylies, with others since discovered,
having special reference to the petrographlcal differentiation of the dyke
rock in passing from the contact walls to the centre of the dyke. The
fact that dykes are very commonly fine grained at their margins and
coarse grained in their middle parts is familiar to all geologists. On ex-
amination of the dykes in question, it became apparent that this varia-
tion in the pliysical appearance of the dykes is not simply one of texttire
or degree of coarseness of the constituent minerals, but that it is rather
the incidental concomitant of important structural, mineralogical and
chemical variations which appear very constantly in the same way in dif-
ferent dykes. These variations are chiefly as follows: Structural — the
passage from the structure of a very fine textured diabase-porphyrite at
the contact walls through the characteristic ophitic structure of diabase
at a few feet from the contact to the granular structure of gabbro in the
middle part of the dyke (Illustrative drawings were submitted.) Min-
eralogical — the passage from a quartzless rock at the contact to a quartz-
ose one towards the middle of the dyke. Chemical — the passage from a
more basic rock near the contact to a more acid towards the middle. The
results of complete or partial analyses by Mr. Shutt of series of speci-
mens taken across a number of dykes were given in tabular form.

The principal object of the paper is to adduce specific evidence that
from a rock mass which is a geological unit of very limited extent, there
may be taken specimens which under current methods of classification
would receive diffe.ent names and be relegated to different classes. The
fact that a series of specimens, in any given locality, differ frc n one an-
other texturally, st/ucturally, mincralogicaliy and chemically, is no proof
that they are not geologically the same rock crystallized from the same
magma. The regular textural and structural differentiation of the dykes
from wall to middle is inferred to have been caused by the difl'erent rate
of cooling under constant pressure. The chemical differentiation Is prob-
ably due to a seleccive crystallization of the more basic minerals in the
earlier stages of solidification accompanied by the transference of acid
residues from the sides to the middle by the agency of Included water.

SALEM PRESS PUBLISHING AND PRINTING CO.

I From the Proceedings op the American Association fob the Advancbmbny

OF iiOIENCE, Vol. XXX Vm,]

NOTK ON THK MAPPING OF TUB AUCH^AN NORTHWEST OF LaKB SUPE-
RIOR. By Dr. Andrrw C. Lawson, Ottawa, Out.

[abstract.]

The writer exhibited a general geological map of the Archaean country
between Red River Valley and Lake Superior, showing the results of re-
cent investigations which he has been conducting for the Geological Sur-
vey of Canada. The proof of a new geologically colored map of the
Rainy Lake region, shortly to be issued by the survey, was also exhibited
to illustrate the details of a portion of the general map. The relative
distril)ution of tin; Upper and Lower Archaean, as displayed on the map,
was shown to be peculiarly Interesting and instructive. The Lower Ar-
chaean or Laurentian, consisting of various, more or less foluited granites
and syenites, which have hitherto been regarded as the oldest rocks, was
shown to occupy lai-ge, isolated boss-like areas which appear to be Ir-
ruptive or intrusive through the schists. The general mapping of this
portion of the country, where denudation has left the Upper and Lower
Archaean In nearly equal proportions, strongly supports the view that the
Laurontlan rocks are of later age than the schists of the Upper Archaean
and were irrupted through them.

The importance of careful mapping as an aid to the solving of the pro-
found problems of Archaean geology was dwelt on, and other evidence
which the writer has adduced elsewhere In support of the irrupt've nature
of the Laurentian was referred to.

Salem Press Publishing and Printing Co.