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 NOTES ON A FEW DYKES CUTTING LAU .ilNTIAN 
 ROCKS, MORE ESPECIALLY WITH REFERENCE 
 TO THEIR MICROSCOPIC STRUCTURE. 
 
 IW B. J. Hahhk.gton, 
 Of the Geological Survey of Canada. 
 
naiiN.ili ( 
 
C.rm N.il i r.iMil _^»'i Sit \ ol VIII 
 
 'L.tr III 
 
 ¥^SA< 
 
 h. 
 
 . • 
 
 B T II ,iel 
 
 TJic Burlwiid Ufslmrafs luht'^Mitrilff.ti 
 
,» ^ 
 
 .yV***.<V*' 
 
 
 .'/ 
 
 ',■"*■ 
 
 
■ V,' 
 
 "5 
 
 (From llir ^^ Canadian Xaturalinl^ Vol. Vllf^ No. (>.'' ) 
 
 NOTP]S ON A FEW DYKES CUTTING LAURENTIAN 
 ROCKS, MORE ESPECIALLY WITH REFERENCE 
 TO Til KIR MICROSCOPIC STRUCTURE. 
 
 By B. .1. Harkincton, 
 Of the (ic'iloi^ical Survcj (if Canada. 
 
 
 
 Tlie fact tliat roeks of L lurctitiiin ai^e are froqueully cut by 
 trap dykes, was many yoiirs ago noticed by Sir William Logan, 
 who traced out and mapped a number of those found in Gran- 
 ville and some of the u' ighbouring townships. Since then other 
 observers have noted tlieir occurrence in widely distant Ijauren- 
 tian areas. IMr. Vennor, for example, observed dykes in Madoc 
 and North Burgess. Mr Macfarlane in liis report on Lake 
 Superior * describes dykes which cut the Laurentian rocks at 
 Goulais Bay, Gros Cap, and other localities, and from his de- 
 scriptions some of them appear to resemble those found in Gren- 
 ville. At Goulais Bay they are from nine to seventy feet thick, 
 strike N. 72° to 75° W., and are probably doleritic. Others at 
 Gros Cap and the mouth of the Montreal River Macfarlane also 
 considers to be dolerites, but states that near Michipicoten Har- 
 bour, and in Bachewahnung Bay, there are dykes of diorite. 
 He further states that at two diiferent points in the Laurentian 
 area examined by him, he observed intrusive rocks of the 
 character of the '• newer traps or melaphyres whicli characterise 
 the upper copper-bearing series." 
 
 • Geology of Canada, 1866, p. 120. 
 
 I 
 
Professor Boll, of the Goolo-ical Survey, has rcpeatcaiy noticed 
 the occurrence of d^kcs in the rc<,'iona explored by him north of 
 l.akes Superior and Huron, and states tliat in some parts of the 
 country they form a conspicuous feature in thcpoolo<;y, and liave 
 probably ph.yed an important part in producinj,' the present 
 j;'!Ographieal fiatures. One described by him as a diorite in 
 the report of the Survey for 1875-7G (p. 'VA) is said to be from 
 oOO to 100 feet in width. Its course is N. l^'' W., and it cut.s 
 a thinly bedded micaceous L'lieiss nearly at ri-;ht angles to the 
 strike of the l.itter. 
 
 Mr. G. jM. Dawson has :il-o uiven us a nund)er of facts con- 
 cerning dykes at the J.ake oi'the Woods, where they are said to 
 be bolh -ranitic and dioritic. Some of the l.-.tter, which are 
 eoarsc-f-rained and app:.re,itly have -. i.eral e.ist and west courses, 
 '•may very probably be amon;,' the oldest of tlie intrusions."' 
 There are others, however, whieh an- very hard and compact. 
 and have a general beiirin-i ol' north-east and south-we.st. These 
 »!ut not only the intrusive -rat.ites ..f the re-ion, but also the 
 altered Laurentian str;ita.='= 
 
 The late Mr. W.ilter JleOuat lias mentioned the occurrence 
 of dykes of "diorite" from fifty to one hundred feet thick at 
 several localities between lakes Temiscamang and Abbitibbe, and 
 states that the apparent direction of two large ones on Lac des 
 Quinze (on the Upper Ottawa) is not tli by-east a.id south-by- 
 wcst. f 
 
 Tt is therefore evident that in aliuost all parts of the country 
 where the Laur«intian rocks have been examined, they have been 
 found to be cut by dykes of various intrusive rocks, few of which 
 have, however, been eritic;illy studied as yet. 
 
 The intrusive rocks of the Grenville region are of special inte- 
 rest, inasmuch as most of them were shown by Sir William Logan 
 to belong to r. date anterior to the deposition of the Lower Sil- 
 urian. According to the descriptions given in the Geology of 
 Canada, they consist of doleritc, syenite and felsite porphyry. 
 Of these the oldest '• are a set of dykes of a rather fine-grained 
 dark greenish-grey greenstone or doleritc, which weathers grey- 
 ish white." =!= 'fi ^l-' " Their width varies from a few feet to 
 
 « See Iteporl oti tlie (Jeolo-y iiiHJ Ittsoiirees of tlie K(\nion in the 
 
 
 vKituly (iC tile Korty-uintli I'juallei. IS?,"), np. '25 r.3, 
 
 t ILjiort (.r J'ro-ress, (Icdl. Siiivry, ISTli-TS, [>[■>. 120, 1 'J2 and 1 
 
 30. 
 
a hundred yards, and tlioy posscsn a woll marked cohuunar 
 structure. Their general benrinj: appc trs to appro icli oast and 
 wet<t, but the main dyk(!S occasion;! I ly divide, a branch strikini: 
 off at an aniile of tVon) twenty to forty decrees. " Son.!- of thcni 
 have been traced tor many miles, cuttinj: both the limestones and 
 gnei.s.ses, and sometimes formini: a rid^e across the limestone and 
 a hollow in the ;:neiss. Whenever they are seen to come into 
 contact with the .syiuitt- they are 'nterrupted or cut oti" by it, 
 belnii therefore more ancient; and 'the relations" Sir William 
 states '-of till' base of the liower Silurian jrroup .liong the foot 
 of the hills composed ot the .syenite are such as to make it evi- 
 dent that the Silurian beds in some pl.ices overlie eroded portions 
 of the intrusi\e rock." All the intrusive rocks of this reuiou 
 are, however, cut by a set of dykes the relations of whicli to the 
 Silurian series is not known. They were described in Sir William's 
 orijiinal report under the name of melaphyre, but were after- 
 wards de>i;j:nated by Hunt as dolerites. thou<;h differinj^ con 
 siderably in characters from the older rocks of that name. 
 
 The writer reirreis that he has not had an o{>portunity of 
 visitiui.' any of the places mentioned above, or even of .seeinji 
 authentic specimens of any of the dykes, with the exception of a 
 few from Greuville and two or three other localities. These 
 specimens have, however, been sliced and studied microscopicully, 
 and a few notes on their miero-copic ch iractersmay be of interest. 
 
 MlOROSCOl'ir dllAKACTERS. 
 
 I. Orntoil/e, Lot 9. Rmigi IV. (Plate, fi-. 1) The ex- 
 amination of a specimen from this locality shows it to consist of 
 plaj^ioclase feldspar, augite, majinetite, viridite/'= apatite, and a 
 little mica and iron pyrites. The pla<;ioclase forms a very con- 
 siderable proportion of the rock, and although much of it has 
 undergone alteration and lost its transparency, it still shows in 
 places, with polarised light, the banuod appearance common in 
 plagioclastic feldspars. It has evidently crystallised before the 
 augite, as blades of it are frequently seen to penetrate the latter 
 mineral. The augite is pale brown or in plaees pinkish in colour. 
 Its form has, for the most part, been impressed upon it by the 
 
 * This useful iiamo i.s applied to a number of green substances 
 which often re.suU fiom tJie decompuBition of augite, liornl.ltnde and 
 ohvim , and whicli cannot uiways be ''individualised." 
 
(ttluT luinorals, but licrc and tlicro u rude crystiil may be observed. 
 The ?>»ica is prf'sont in small (juantity, and is biown and strongly 
 dichroio. Magnetite (possibly titano-ferrite) is abundant, owur- 
 in<r ('lii('% in irreuul.irly sliapod <rrains, but sonictinns showini: 
 rude ()ctiili('(liMl form. Sonn'tinu's it is scrn in innuuu-rablo small 
 trrsiiiis ijnboiMcd in the au'zitc. Tlv viridite is .ibutidrmt and 
 very bright i;rct'n. It occurs iar^t'ly in Hbrous or sbo:if-liko a^ 
 irr('i:ations slmwinii faint dicliroism. and with the polariscopc; 
 chan^iny, on rotation of the analyser, from bhio to brown. In all 
 probability it i^ chloritf. The apatite is tound in sharjdy detinod 
 acicnlar crystals whitii are liexau'cuial wlivn .'<oen in cro.ss section. 
 It is most abundant in the feldspar, but is also seen to p*>netrate 
 the mica, auiiitc. ^md even tin' luaiiiielite. 
 
 11. (ir,nrillr. Lot !». linmf: V. \\ : on examined with the 
 micru.scope the section of this r(»ck is. like th.ir list described. 
 seen to eonsi.-t of pl;i<:ioclasi', auuite. mamietite, viridite, pyrite 
 and apatite? Tin; feldspar forms a network of blades, and has 
 in places undergone some aUenttion. althoui:h for the moKt part 
 it appears to be uiinltered and with the pol.iriscope becomes 
 beautifully banded. It is di.stiiu'tly seen to penetrate the mai-- 
 netite in a number of instances, and must therefore ha\e solidified 
 before, or -it least simultaneously ^vitli the magnetite. It also 
 contains a L!;ood many of what .ippear to he iilass- and stone- 
 cavit.ies. The auujite is brownish ^rey in colour, traversed by 
 numerous fissures aiui penetrated iii all directions by blades of 
 feldspar. The rock cotitains a ^ood deal of magnetite, mo.stly 
 in <;rains of irreuular form, but occasionally in octahedral cry.x- 
 tals. When cut acro.«<s the grains are often seen to eoTitain 
 numerous irrcuular cavities. ,ind in one case .in octahedral crystal 
 was observed which was hollow, or nothinu more than a shell. 
 Viridite is present in considerable (juantity. It is much duller 
 green than that in the rock last described, and looks more like an 
 alteration product -.f the augite. It is mostly amorphous, but 
 occasionally occurs in sheaf-like 'ggregate Pyrites is present m 
 small irregular grains scatteri'd here; and there through the rock.-"*^' 
 
 * Spciiiiicns T and II were many years ii,t;<> analyse'l l)y Dr .llnnt, 
 who di'seribed tlieni as follows: "'flic dyl-es of tin's most undent 
 dolerite or greenstone in (I'renvillc have a well-marked eolnmnar 
 stnietiiro at riL';lit joiales to the plane of the dyke. They are line- 
 jrrained, dark greenish-gray in eolor. and weather ;j-rayish-white. 
 
 i 
 
observed, 
 il stron'j'lv 
 >nt. occur- 
 « showin,:^ 
 .iblo small 
 id.-int and 
 if- 1 ike .'1^ 
 •olariscopc; 
 n. Ill all 
 »ly (letinod 
 88 section. 
 I penetrate 
 
 with the 
 (U'scribt'd. 
 lite, pyrite 
 ;s. and has 
 most part 
 e becomes 
 
 the niML-- 
 e solidified 
 It also 
 mid stone- 
 versed by 
 
 blad's of 
 ite. mostly 
 'dral crys- 
 to contain 
 ral crystal 
 i:in a shell, 
 ucli duller 
 ore like an 
 phous, hut 
 > present in 
 the rock.-"*^' 
 
 V Dr .Hunt, 
 Dst iUK icnt 
 I columnar 
 ey arc line- 
 ivisJi-white. 
 
 5 
 
 III. I^iitr St. Simon. (Plate. ti<j;. 2.) This .specimen is 
 hoin a tine jrruinod, <rreyish-black dyke whieh cuts the Oreoii 
 J.ake band (tf crysiuUiiK li»'.iestone on the '^t. Siuu J. a small 
 tributary <»f the Nortli River, in 'i'trrebonne County east of 
 (Jreiiville. I iiui indebted f«»r it to the Director oi' the (jeokv^i- 
 e.d Survey. The dyke probably belongs to the s.-Mue set as the 
 (jrenville ones just described, its «j;en«ral structure beiuL' the 
 same, but it has apparently undorjione very little alterali(»n. tin- 
 section beiiii; beautifully clear and tr;;iispari'iit. With the micro- 
 .scope it :s seen to consi.-t (tf a network of plaj^iodate feldspar, 
 v;ith auuite, magnetite and apatiu- (?) and a very little viridite. 
 The teldspar as seen in the section is perfectly transparent and 
 colourlesis, and with tht! polariscope shows a beautifully banded 
 .•*tructur«'. In places it contains microlites whicli arc possibly 
 apatite, and also „ few vapour- or gas-cavities, generally ingruup.s. 
 The augitc is \)nU'. gieyishbrown frecjuenti, penetrated by 
 blades of feldspar aiid often containing groups of minute grains 
 of magnetite. It appears to constitute about half the rock. 
 The magnetite occurs mostly in irregtdar grains and misses of 
 n.ost fantastic si; pe. but now and then in rude crystals and rod- 
 like forms. In some cases it is seen to be penetrated by blades 
 of feldspar. (kSee tigurtss on next page.) 
 
 The viridite is not very abtiudant and looks as if d( rived from 
 
 Undtr ;i liiis tlic ruck i;; si( ii to consist of a gni-nish-wliitu tVItlspiir 
 
 witli ii si;ily tVaiUut . niinirlt-d vvitli j;raiiis of pyroxiuc, ocfiisioiial 
 
 plates ol inieu, aiul grains ol pyriti-s. It roiitains im iiiri)onjiti «. 
 
 Tv/u Jinalyscs ol' i)ortions ol tlic (loK-riti- troni (lyU< .-< ditVi liuK a iittli' 
 
 in texture gavr as follows: 
 
 Silica ;j(».3j 50.2'i 
 
 Aliiniina I l.'M} y^ .,., , ^ 
 
 Pcroxyd ol iron 1 '-'..'jo j 
 
 Lime lo.llt 0.63 
 
 Mat,'ncsia 4.03 5.04 
 
 I'oliish <Jit ..'iH 
 
 Sotla i:.2H 2. 1'.' 
 
 Volatile Ta l.nn 
 
 yj.iM 100.72 
 
 •' Tlie iron in thtsc analyses, althougli ,uiveu above as jjuroxyd, 
 t xists in the form ol' protoxyd, and in tlie second specimen, in part us 
 a sulpluuet." (Am. Jour, ol' Sci., 18(54, 2nd Ser., Vol. .xxxviii, p. 174) 
 V.'liicli of the analyses applies to the .ipecimen from Kangt; IV jitul 
 which to that from Uantre V is not stated. 
 
the augite. It is rjither dull green and can scarcily be said to 
 exhibit dichroisni. In pljteejs it hhows numerous fine lincii! run- 
 ning in seveml dirf r tions. 
 
 y. 
 
 .? 
 
 mimiK til.- p. lulrati 
 
 lil;u1fs of 
 
 Fiyiiies 1 ;iii<l •>. — (iiMin> 
 
 Kiffiirt' 3. — <!r«>iip slinwiim a f,\v nf tlir vfirii'd f'oriiis wliifli tlio 
 inji<;ni'titc a>siin)c s. ( x 78.) 
 
 IV, Rivddiigvnn. Tcrnhoinie Coiitif}/. (I'late, tii:. 3. ) The 
 •specimen IVuni this locality is coarHor in texture th:iii the last, 
 and of a dark grey colour. Its specific gravity is 8.01 :i. The 
 dyke whore observed by Mr. SelwyT) (to whom I am indebted 
 for rlie specimen ) cuts a b.-md of gneiss, and is in .ill probability 
 of the same age as thf Grenville ones, though it has not been 
 traced out. T!:e exumination of a thin stetion of tlie rock 
 shows it to be composed of pl.igioclase feldspar, augitt-, .iagnetite, 
 .•>patite and a little niic;- and viridite. The plagioclase shows 
 «'videne(! of but little alteration, and much ol' it is striated as in 
 the ease of the River 8t. Simon joek. and with polarised light 
 beautifully banded. The blades run in all directions, but do not 
 constitute as continuous a oi'twork as in the last specimen, since 
 the augite is much more abuodant. Blades of the feldspar 
 frequently penetrate the augite. and occasionally al.so the magne- 
 tite. The augite is pale blown in colour, perfectly fresh, and 
 often dotted with what .ippe.ir to be g.is-or vapour-cavities. Its 
 cleavage is often woll-markcd atul it occasionally shows twiiniing 
 (see figure). The magnetite is not V(;ry abundant and occurs in 
 irregular and often fauta.^^tic I'.rms. The apatite and mica are 
 present in very small quantity, as is also the viridite. The 
 latter cViefly accompanies a brown somewhat decomposed mineral 
 which has not been determined. With polarised light the section 
 forms a beautiful object. 
 
 J 
 
' be said to 
 le lincii! run- 
 
 y. 
 
 • 
 
 iiy lil;u1fs of 
 ' wliicli tlio 
 
 i-. 3.) The 
 ;in the last, 
 i.Oi:i. The 
 111 indebted 
 
 probability 
 Hs not been 
 •I' tlie rock 
 , -.iti^iinetite, 
 clase shows 
 tri.ited as in 
 ■•irised lijjht 
 . but do not 
 ;'iinen, since 
 the feldspar 
 
 the na;ijj;ne- 
 y fresh, and 
 ivitii's. Its 
 ws twiiMiinsi; 
 id oeeurs in 
 1(1 mica are 
 idite. The 
 ised mineral 
 : the s(^ction 
 
 J 
 
 V, GrenviUe, Lot 4, Range VI. The specimen from this 
 locality is from one of the newer dykes which, as already stated, 
 cut all the other rocks of the region. It consists of a dark grey 
 fine-grained base (sp. trr, 2.83) with occasional jtorphyritically 
 imbedded masses of hornblende, which are '>ften accompanied by 
 a plagioclase feldspar, Calcite is also present in white cleavable 
 masses, mostly filling cavities* 
 
 Microscopically this rock is very diflFereut from those already 
 described, but it requires much fuither study. Tlif* ground-mass 
 appears to consist of a mixture of plagioclase, biotite (very ttbun- 
 dant), and magnetite or titano-ferrite, with a good deal of a green 
 mineral which is probably an alteration product, and is not at 
 all dichroic. Here and there also thi-re are almost colourless 
 crystals, which may prove to be olivine, very much cracked, and 
 often converted along the cracks into a pale green mineral. As 
 stated above, the rock is porphyritic, and a section cut across 
 one of the porphyritic masses shows it to consist of beautifully 
 striated plagioclase with embedded crystals of liorn blende and a 
 little pyrite, while all these three miuera s contain numerous 
 crystals of apatite, the largest cross sections of which measure 
 about 0.25 mm. Some of the cross sections are perfect hexa- 
 gons, but none of the crystals when viewed longitudinally show 
 perfect pyramidal terminations, but are generally rounded as seen in 
 figure 6 c of the accompanying plate. When examined with a high 
 power, most of them are seen to contain numerous cavities, which 
 in a few instances have been observed to contain bubbles, although 
 most of them appear to be empty. Almost without exception, 
 too, they contain black globular and sub-globular bodies (see 
 plate), which possibly take the place of the thin nail-like bodies 
 often found in the apatite of basalt. Some of the crystals contain 
 
 * An analysis of this rock was j)iil>lisht<i l)y Dr. Hunt in the 
 (Jcology of Canada and also in th«' Amdicnn .Jounifil of S('it'U<e 
 (8ec«iui Scries, Vol. XXXVlll., p. 174) from whiclitii. following is 
 oxtrartud : *' Wlini in powtlcr the rock <<hrvcsccs trcdv in flic cold 
 with dilute nitric ucid, and the scdiitioii tvolv( s red funies on heatinfi;^. 
 In tills way there were dissolved, lime, ((iiml to M.7'i jior <-ent. of 
 (.■irhona.e, 0.50 of magnesia, and ti'iO of .-ilnniina and oxyd of iron 
 = 15.70 i»er cent. The residue drhd at 'J12" K., eijujillrd S3. 80 per 
 < cnt. A portion of aluminous silicate had evidently hccn attacked 
 by Ihe acid. The dried residue gave ou analysis, silica r)'2,20, alum- 
 ina 18. 50^ peroxytl of iron, with some titanic aciil, lo.oo, lime 7.34 
 magnesia 4-17, potash 2.14, soda •2.41, volatile 2,50 = 99.26." 
 
8 
 
 only only one or two of tliese, but as many as nine have been 
 observed in one case. The rude crystals of apatite which are 
 associated with pyrite arc cracked acroste. and the ciacks tilled 
 with pyrite as shown in ti<>ure 5. 
 
 The aniytidules have a lining oi' a i>reen structureless ujineral 
 (i^reeu earth) while the interior is filled with a colorless mineral 
 which appe.irs in most cases to br calcitc. In some cases also 
 twc cavities contain pyrites, mostly at the junction of the calcite 
 ami green e.irth. 
 
 VI. Mail„i-. Outano, lot 24, Range VI. (Plate, fig. 4.) 
 This rock m.iy be noticed here as u good example of a diorite. 
 It was given to me by x\Ir. Vennor of the Geological Survey, 
 and stated to have been broken from an undoubted dyke. It 
 was supposed to be a pyroxenie rock, but the microscopic study 
 of a thin section shows it to be a diorite, consisting chiefly of 
 feldspar, hornblende and magnetite, but also containing cubical 
 crystals of iron pyrites and small (juautities (»f a transparent 
 mineral which is probably quartz. The feldspar is a good deal 
 altered, but apparently all plagioclase in the sections examined. 
 The hornblende is of a rich green colour, and much of it shows 
 cleavage lines very distinctly. It is dichroic and polariaet. 
 beautifully. In places it appears to have undergone some altera- 
 tion, though not to the same extent as the feldspar. 
 
 Conclnsinns. The first of the rocks just described, on ac- 
 count of the lirgi^ proportion oi viridite which it contains, and 
 the altered state of the feldspar, would be called by German pe- 
 trographers a diabase. One would also expect to find a larger 
 proportion of water than is indicated by the analysis, [n many 
 respects it agrees with Senftfr's descriptions of diabase from the 
 Duchy of Na-sau in Germany. The alteration which it has un- 
 dergone, however, is lot nearly as marked as in many diabases 
 from much youngei- formations, as. lor example, tht- Cretaceous of 
 British Columbia. Much of the viridite looks as if it had been 
 one of the original constituents ol' the rock, but in other places 
 it is pretty evident that it has been derived from the augitc. 
 No. II may perhaps also be called a diabase, althouirh very 
 little removed from such rocks as III and IV. Its general 
 structure is the same, the only important difference being the 
 development of a good deal of viridite. Nos. Ill and IV are 
 tru(> dolerites or ' feldspar basalts,'" iudistinguishable from many 
 
le have been 
 ; whicli are 
 otacks tilled 
 
 LiloHs ujiricral 
 :less mineral 
 le cases also 
 if the calcite 
 
 ite, fi<r. 4.) 
 if a diorite. 
 ical Survey, 
 1 dyke. It 
 ■*copic study 
 «; chiefly ol' 
 uinu, cubical 
 transparent 
 a good deal 
 s examined, 
 of it shows 
 d polarises; 
 ;ome altera- 
 
 bed, on ac- 
 •ntains, and 
 Grerman pe- 
 nd a larger 
 [u many 
 se from the 
 1 it has un- 
 uy diabases 
 ■etaceous of 
 it had been 
 )ther places 
 the augitc. 
 lOUirh very 
 Its general 
 being the 
 lud IV are 
 irom many 
 
 9 
 
 of Tertiary .age. They are highly crystalline and do not appear 
 to contain any glassy base. As yet no olivine has been observe<l 
 either in them or the diabases, but very few sections have been 
 examined, and possibly it will be found on further study. In 
 No. I, a mineral has been observed with the characters of sanidin, 
 and no doubt other minerals will yet be detected. 
 
 The order in which the different minerals have solidified is a 
 matter of interest, apparently not being tiiat of the fusibilities 
 of the constituent minerals before the blowpipe. Fn the diabase 
 and dolerite it is evident that the apatite has been the first to 
 solidify ; the plagioclase appears to liave come next, then the 
 magnetite, and last of all the augite. Mr. J. Clifton Ward 
 gives an interesting example of the apparent order in which 
 the minerals constituting a leucitic basalt near Naples have 
 solidified, which may be noticed in this connection. The 
 minerals are Icucite, magnetite, magnesia-mica, feldspar and 
 auuite. Of these five minerals the only infusible one is the 
 leucite. and yet Mr. Ward thinks that the last four •' were held 
 in solution by leucite in a state of fu-'on : and that instead of 
 this mineral crystallising out first, it deposited in succession the 
 magnetite, the mica, the feldspar i^nd the augite, and last of all 
 probably solidified quickly, enclosing within its crystals glass- 
 and stone-cavities, and magnetite and feldspar cryslals," 
 
 It is evident that No. V is a very diftercnt rock from any of 
 the others described. In some respects it resembles the so-called 
 melaphyres. but contains much more mica than is found in any 
 of which I have seen descriptions. No. VI is as already stated 
 a diorite and needs no further remark here. 
 
 The slight amount of alteration exhibited by some of the 
 ancient dolerites in the Grenville region would no doubt be sur- 
 prising to some, but is not so .uuch to be wondered at when wc 
 consider that they occur in highly crystalhne rocks, which would 
 serve to a great extent to protect them from the agencies which 
 have brought about decomposition in dykes cutting the unaltered 
 strata of some more recent formations. 
 
 <.Jiiart. .lour, (ii'ol. Soc . 1875, p. 3Wi. 
 
1(» 
 
 L)K!S(;i;li'TI(>.\ OF I'LA'l'E HI. 
 
 Fig. 1. l>iiiliiist rumi (.ircii villi, lot y. range IV, showing aiigiti? 
 l>lap<j(Jii,sr, luaiiihtitc anu viiiditi- (umgnifiud "JB diani- 
 i-Urs). 
 
 Fig. li. Dokiitr (Feldspar Jia>alt) Ironi J;iv(i St. iSiiuou, .sLuwiug 
 auyitt.', plagioclasi . niagU( liti ai/d a litth vindite. Tho 
 i-rucit'orm group in tt^ right liand nppei' lormr is [ilagio- 
 tlasf. (X 18). 
 
 Fi(!. 3. Doliritf Ironi Kivrr (iaunon. slj 
 
 owing Hiigitc (a twin on tin 
 
 li'lt) plagUK last- ami magnttitr. (x 14). 
 
 Fio. 4. JJiorit.- Ironi Madoc, Ontario, slii)win<; bluisli-grtt-n I 
 
 K>rn- 
 
 l>lt tide, plagioclasc, jnagnctitc. am! pyritc (the square 
 crystal in tli.' Jowii right hand rcirm'r>. (\ 78). 
 
 F[(i. 5. Apatiti in rork from (irrnvilK-, lot 4, range VI. The portion 
 <>i' the drawing shaded Idack, f.\cfpting tlu; spots in tlm 
 apatite ( j\ stals. ( onsisis of niagni'titc and pyritf. chiefly 
 
 th 
 
 e latter. (.\ 78) 
 
 Fi(3. (J. (((). Cross S( etn)n of a[iatite erystai with nunrerou.- cavities 
 a tew of wliich show bnbhle.s;^ and are perimps liquid 
 ca\ities. (/)). Cross section of apatite crystal, showing 
 the black bo<li(s leftrn li to in the text. (r). Longitudinal 
 section of rounded apatite erystai with black bodies .sim- 
 
 ih 
 
 to those in 
 
 ( 
 
 .\li \ 78.) 
 
twill on the 
 
 rttii liorn- 
 tlic sqiiarf 
 
 riie portion 
 *pots in thd 
 ritf. chiefly 
 
 (i JS (iiani- 
 
 'U, .sLuwiug 
 iclite. The 
 
 I is [iliiuio- 
 
 iis caA'itios 
 laps liquid 
 I, sliowing 
 :)nt;itudin;ii 
 Hxlii's sim-