72 ROCK-METAMORPHISM. 



CHAPTER XII, 



ON THE ORIGIN OF THE ARCHAEAN " CRYSTALLINE 

 LIMESTONES " OF CANADA. 



THE Canadian Archseans are for the most part mineralized 

 metamorphics, the methylosed members (usually silo-carbacid) 

 being a subordinate group. In general highly crystalline, the 

 former consist of bedded masses of granitoid gneisses,, quartzites, 

 diorites, dolerites, various crystalline schists, &c., and the latter 

 of calci-hornblendic gneisses, ophites, " crystalline limestones," 

 and other related kinds. The " crystalline limestones " having 

 been proved to be chemically, mineralogically, and structurally 

 identical with hemithrenes, we shall assume them to be rocks 

 of the latter kind. 



The " crystalline limestones " or hemithrenes, which are 

 often interstratified with the mineralized metamorphics, vary 

 much in their mineral composition : calcite, or miemite (either 

 of which is usually present) generally serves as a matrix for 

 the mineral silicates chondrodite, augite, hornblende, phlogo- 

 pite, orthoclase, labradorite, serpentine, quartz, idocrase, &c.; 

 which, with apatite, graphite, and other non-silicates, occur as 

 crystals, or as irregularly-shaped grains (crystalloids) varying 

 much in size, " either alone or variously associated, and some- 

 times in such quantities as to make up a large proportion of the 

 rock, to the exclusion of carbonate of lime /' so that beds are 

 often seen to graduate completely into diorites, gneisses, and 

 other silacid metamorphics *. 



The beds are often greatly contorted ; and their component 

 layers are frequently and independently crumpled in the most 

 extraordinary manner. At the Ragged Chute on the Mada- 

 waska (Canada) there is a bed, according to Logan, " three feet 



* Report of the Geological Survey of Canada for 1866, p. 185; and 

 Chemical and Geological Essays, p. 206, 



