META-IGNEOUS ROCKS 461 



various devitrified volcanic rocks which, following the proposal of Doctor 

 Bascom,^^ are known as apobsidian, aporhyolite, etcetera. Other igneous 

 rocks, chemically altered under mass-static conditions and designated by 

 the prefix "apo," also belong in this category. 



Meta-sedimentary Eocks 

 foliates 



Para-gneisses and schists. — Eocks belonging to this important group 

 of para-foliates are produced from strata essentially by pressure under 

 either dynamic or mass-static conditions and usually with notable recrys- 

 tallization. Except in a very few obvious cases — for example, conglom- 

 erate gneiss — the term "gneiss" or '^schist" should be immediately pre- 

 ceded by the prefix "para-" to indicate derivation from a sedimentary 

 rock, and this in turn by the names of the principal minerals contained. 

 A great variety of schists and gneisses are to be thus classified, and in 

 many cases there appears to be no satisfactory way of designating these 

 except by long names. 



Two important types of these para-gneisses and schists should be recog- 

 nized: (1) dynamic para-gneisses and schists, which are sedimentary 

 rocks with foliation developed essentially by compression accompanied by 

 crystallization and usually with destruction of stratification, and (2) 

 static para-gneisses and schists, which are sedimentary rocks whose folia- 

 tion has been produced essentially by crystallization under mere down- 

 ward pressure, due to load of overlying material, and often with more or 

 less well preserved stratification. The writer is strongly of the opinion 

 that static para-gneisses are much more common, especially ^mong the 

 very ancient rocks, than has been recognized. Such rocks have recently 

 been described by Daly*^ in British Columbia and by the writer*^ in 

 northern New York. 



Para-slate and para- phy Hit e. — These are metamorphosed sediments, 

 usually shales, with highly developed foliation produced under dynamic 

 conditions, but with little or no megascopically evident crystallization. 

 Thus their process of development and general appearance are much like 

 those of the ortho-slate and ortho-phyllite, but they are notably more 

 common than the latter. Since the para-phyllites exhibit all stages of 

 transition between the true slates and the true schists, it becomes a matter 



39 F. Bascom : BuU. Geol. Soc. Am., vol. 11, 1900, pp. 121-122. 



*" R. A. Daly : Geol. Surv. Canada, Transcontinental Guide Book 8, pt. 2, 1913, pp. 

 131-132. 



"■ W. J. Miller : .Tour. Geol., vol. 24, 1916, pp. 588-600. 



