188 THE CANADIAN NATURALIST. [June, 



16 5 of alumina. These, in the vicinity of the dolerite, have be- 

 ccne satiir ited with protoxyd bases, includino; the small portions 

 of magnesia an! of oxvd of iron which the limestone contains. 

 This )rocess evidently involves a decomposition of the carbonate 

 of lime, a-'d the expulsion of the ca bonic acid. It is worthy of 

 remark that while the unaltered limestone contains a little car- 

 bon ite of ma2;nesia, the rock from which iii was obtained yielded 

 to di'ute nitric acid not a trace of m ignesia. ii marks an inter- 

 mediate staL'e in the process, and shows moreover that the alkalies 

 are still retained in combination with the alumonius silicate. 

 These granuhir silicates, which have been formed by local 

 metamorphism, might, under favorable circumstances, have crys- 

 tallized in the forms of feldspar, scapolite, garnet, pyroxene, 

 or some other of the silicious minerals which so often occur in meta* 

 morp ic limestones. The agent in producing these silicates of 

 protoxyds at the expense of the carbonates of the limestone, was 

 probably a portion of alkaline salt, either derived from the feld- 

 Bpathic matter of the limestone, or possibly infiltrated from the con- 

 tigu )us feldspathie rock ; whose elevated temperature produced the 

 reaction which has resulted in thus alterinj; this limestone. 



Similar examples of local alteration are m t with in several other 

 places near to the intrusive rocks of the Montreal group. The 

 schists of the Utica formation in contact with a dyke of intrusive 

 rock at Point St. Charles, and also near a mass of trachyte on a 

 Smill island opposite the city of Montreal, occasionally exhibit small 

 crystals of pyroxene, and in some cases prisms of hornblende. 

 Among similarly altered shales at Kougemont are beds whicii con- 

 sist of a highly ferriferous crystalline dolomite intermingled with 

 dark-green cleavable h )rnblende, which forms thin layers, or in 

 other cases encloses smdl rounded masses of the dolomite. ^See 

 for a description and analyses of this rock the Geology of Canada, 

 page 634.) 



At Montarville the shales of the Hudson River formation 

 are altered in the vicinity of the dolerite which forms the mass of 

 the mount lin. Some portions of the strata are very fine-grained, 

 reddish-brown, and hive an earthy sub-conchoidal fracture, with 

 occ isional cleavage joints. The hardness of this rock is not great, 

 and it is apparently a kind of argillite; but between two beds of it 

 is one of a harder coarse-grained rock, greenish-gray in color, and 

 mottled with a lighter hue. This appears to be feldspathic in 



