1854.] 



ON SOME OF THE CEYSTALLINE LIMESTONES OF NORTH AMERICA. 



37, 



times dolomitic, and Hitchcock observes that in the granular marbles 

 of Berkshire, pure and magnesian limestones occasionally form dif- 

 ferent layers in the same bed. (Geology of Massachusetts, p. 84.) 



In Orange county, according to Mather, it is easy to trace the 

 transition fi-om the unaltered blue and gray fossiliferous limestones 

 of the Champlain division, (including the Calciferous sandrock and 

 the Trenton,) to the highly crystalline white limestone with its 

 characterislio minerals. (See his Report on the Geology of the first 

 district of Xew i'ork, pp. 46-5 and 486.) This view is fully sustained 

 by H. D. Rogers in his description of the limestones of Sussex Co., 

 given in his final report on New Jersey, (cited by Mather, as above, 

 p. 468 et seq.( Mather fai-ther concludes very justly that all the lime- 

 stones of Western Vermont, Massachusetts and Connecticut, and 

 those between the latter state and the Hudson River, are in like man- 

 ner altered Lower Silurian strata, (p. 464.) From the similarity of 

 mineral characters, he moreover supposes that the crystalline lime- 

 stones about Lake George are of the same age, and he extends this 

 view to those of St. Lawrence County. Both of these however belong 

 to the Laurentian series, and are distinguished by their want of 

 conformity with the Champlain division, and by their associa- 

 tion with labradorite and hypcrsthene rocks which seem to be wanting 

 in thij altered Silurian strata. The slates of this division in Eastern 

 Canada, generally contain some magnesia, with very little lime, and 

 four or five per cent, of alkalies, chiefly potash ;* hence the feldspar 

 which has resulted from their metamorphosis is generally orthoclase, 

 - anil they have yielded gneiss, and mica slate, which with quartz rock, 

 and chloritic and talcose slates, make up the Green Mountains. 



In the upper part of the Champlain division, there are found some 

 beds of a limestone, often conglomerate, which is generally mag- 

 nesian and ferruginous, and often contains a great deal of silicious 

 sand ; and associated with it are beds of carbonate of m.agnesia with- 

 out a trace of lime, though sometimes very silicious. These beds 

 are interstratified with slates and sandstones, and in the metamorphic 

 region are replaced by the serpentines, which are often intermixed or 

 associated with limestones and dolomites, and, with their accompany- 

 ing talcose slates, may be traced one hundred and thirty-five miles in 

 Canada, and thence by Vermont, Massachusetts and Connecticut, 

 through New York, New Jersey, Pennsylvannia and Maryland, south- 

 ward. These rocks are everywhere marked by the occurrence of 

 chromic iron ore, in masses running with the stratification, or in 

 disseminated grains, in the serpentine, and sometimes in the dolomite ; 

 they are also the aui-iferous rocks of the great Appalachian chain. 

 Gold, associated with talcose slates, serpentine, chromic and titanifer- 

 ous iron ores, is traceable along their outcrop from Canada to Georgia. 

 Gold-bearing veins have also- been found in the slates which in Eastern 

 Canada, from the base of the Upper Silorian. I remark that in a 

 somewhat chloritic and very silicious magnesian limestone, which is 

 associated at Granby with red and green slates and sandstones, a por- 

 tion of oxyd of chromium was detected by analysis. I have also 

 found titanium in some of the very ferruginous slates, which by their 

 alteration become chloritic schists holding magnetic and specular iron, 

 ilmeuite and rutile. 



Serpentine is found as an imbedded mineral in the Laurentian lime- 

 stones, but the extensive deposits of serpentine rock, with its associated 

 talcose slates and chromic iron, appear to be confined to the upper part 

 of the altered Champlain division. The examinations of C. U. Shepard, 

 and those subsequent of J. Lawrence Smith and G. J. Brush, have 

 shown that many at least of the so-called serpentine rocks of northern 

 New York, are hydrous silicates of alumina, iron, and potash, contain- 

 ing very little lime or magnesia ; they arc the dysyntribite of Shepard. 



As the north-western limit of the metamorphic belt in Eastern 

 Canada runs southwesterly into Vermont, the undulations of the strata, 

 which arc nearly N. and S., escape from it to the northward. Pro- 

 ceeding E. S. E. however, from the unaltered Trenton limestones of 

 the Yamaska, we cross the overlying slates, sandstones, and dolomites, 

 and entering the metamorphic region find the serpentines, talcose, 

 chloritic and micaceous schist, with gneiss and quartzite, very much 

 disturbed, and repeated by undulations. On reaching the valley of 

 Lake Memphramagog, we come upon the third class of crystalline lime- 

 stones, which are Upper Silurian. This limestone formation has a 

 continuous outcrop from the Connecticut vallev, by the lake just men- 



* See my remarks On the Composition and Metamorplioses of some 

 Sedimentary Rocks, L. E. and D. Philos. Magazine for April, 1804, p. 

 233. 



tioned and the upper part of the St. Francis river, to the Chaudifere, and 

 is thence traceable by intervals as far as Gaspi5, where it is clearly 

 unconformable with the Lower Silurian. It holds the characteristic 

 fossils of the Niagara group, but for some distance from the line of 

 Vermont, is so much altered as to be white and crystalline, and to con- 

 tain abundance of brownish mica, the fossils being often obliterated. — 

 At Dudswell on the St. Francis, the beds of white gi-anular niai-ble 

 show upon their weathered surfaces or in polished sections, the forms 

 of encrinal discs and corals, among which the characteristic Fauosiles 

 gothlandica, and various species of Forties and Cyalhoplq/llvm, have 

 been identified. These fossils in a similar condition are also found at 

 Georgeville on Lake Memphramagog. Following the section in a 

 S. E. direction, to Canaan on the Connecticut river, we meet with 

 calcareo-micaceous schists, which are gradually replaced by mica slates, 

 with quartzose beds. Some of the fine dark-coloured mica-slates 

 exhibit crystals of chiastolite, and others near Canaan, abound with 

 black hornblende and small g.ai-nets. (For the details of this section 

 see Mr. Logan's Report for 1847 — 48.)^- 



These Upper Silurian strata constitute the micaceo-calcareous rocks 

 of Vermont, which Prof. Adams traced througli the state, to Halifax 

 on the border of Massachusetts, and they are continued in what Hitch- 

 cock has called the micaceous limestone of this state, which according 

 to him pass by insensible degrees into mica slate. The limestones of 

 Coleraine, Ashfield, Deerfield and Whately, Mass., belong to this for- 

 mation and perhaps also the crystalline limestone which is found at 

 Bernardston, with magnetic iron and quartz rock, and shows imperfect 

 fossils upon its weathered surfaces. (Hitchcock's Geol. of Mass. 

 p. -560.) The condition of these limestones resembles that of the granu- 

 Lar marbles on the other side of the Green Mountains, and they 

 nowhere exhibit that degree of alteration which distinguishes the latter 

 farther south. The same calcareo-micaceous rocks are conspicuous in 

 western Connecticut; but in the towns of Salisbury, Sharon, and 

 Canaan the crystalline limestones, and in Litchfield and Winchester, 

 the serpentines, of the Lower Siliu'ian are met with, and these rocks 

 appear again in the southwestern part of the state. 



In the fourth class we include the crystalline limestone of Eastern 

 Massachusetts, which occurs in a great number of places in the towns 

 of Bolton, Boxborough, Chelmsford, Carlisle, Littleton, Acton, Natick, 

 and Sherburne. It appears according to Hitchcock, in inten-upted 

 lenticular masses, lying in the gneissoid formation, or in the liorn- 

 blendic slates, and occasionally presenting distinct marks of stratifica- 

 tion. Still further east at Stoneham and Newbury, we find crystalline 

 limestone, sometimes magnesian, in irregular masses, lying in a rock 

 intermediate between syenite and hornblende state. Serpentine is 

 found with that of Newbury ; and at Lynnfield, a band of serpentine 

 has been traced two or three miles' N. E. and S. AV. Dr. Hitchcock, to 

 whose report on the Geology of Massachusetts we are indebted for the 

 present details, says of this serpentine, " I am satisfied that it is cm- 

 braced in the great gneiss formation, whose strata run from N. E. to 

 S. W. across the state." p. 159. He further remarks of the syenite 

 of Newbury and Stoneham, which includes the crystalline limestones 

 " I have every reason to believe that it is only a portion of a gneiss for- 

 mation which has undergone fusion to a great degree : for portions of 

 the rock still retain a slaty or stratified structure," and he conceives it 

 probable that all the crystalline limestones of Ma.ssachnsetts are of 

 sedimentary origin; p. 586. It may be remarked that the irregular 

 shape of these interstratified masses, is analagous to the interrupted 

 stratification and lenticular beds, frequently met with in fossiliferous 

 limestones. 



The limestones of Bolton, Chelmsford and the adjoining towns are 

 in general highly crystalline, and are remarkable for the variety of fine 

 crystallized minerals which they contain. Among these arc "apatite, 

 serpentine, amianthus, talc, scapolite. pyroxene, petalite, chondrodite, 

 spinel, cinnamon-stone, sphene and allanite, which include the species 

 characterizing the Laurentian and Lower Silurian nictaniorpliic lime- 

 stones. The limestone of these quarries evolves a very fetid odor when 

 bruised. Chromic iron ore has never so far as I am aware, been ob- 

 served with the serpentines of this region. 



We have now to inquire as to the geological age of this great mass 

 of cry.stallinc rocks which is so conspicuous in Eastern New England. 

 Mr. Logan has shown that the rocks of the Devonian System in Gasp^, 

 assuming the Oriskany sandstone as its base, attain a thickness of more 



* See also on the Geology of Canada, Silliman's Journal [2] vol. ix. 

 p. 12, and xiv. p. 221. 



