META-IGNEOUS ROCKS 459 



morphic rocks, (2) the foliation is impressed on them after nearly com- 

 plete consolidation of the magmas, and (3) they almost always exhibit 

 more or less granulation, it seems clearly advisable to classify the primary 

 ortho-foliates with the metamorphic rocks. Certainly, however, they 

 should not be called true gneisses, which always ought to be regarded as 

 of strictly secondary origin. Following the suggestion of Gordon,^* the 

 structural term "gneissoid,^' meaning literally "resembling a gneiss," is 

 applied to these rocks. Thus we have gneissoid granite, gneissoid diorite, 

 etcetera. In rare cases, where the gneissoid structure is so perfectly de- 

 veloped that the rock cleaves more or less readily, the term "schistoid" 

 may be employed. Fine large-scale examples of gneissoid granites have 

 recently been described by Adams and Barlow^^ in Ontario and by the 

 present writer^^ in the Adirondacks. 



Secondary ortho-gneisses and sc/iisis.— These are so named because 

 they are crystalline, foliated, igneous rocks of strictly secondary origin. 

 Foliates of this kind are produced essentially by pressure, often accom- 

 panied by more t)r less recrystallization. In by far most cases the folia- 

 tion appears to have been produced under dynamic conditions, but recently 

 some igneous rocks have been described as having had their foliation 

 developed under mass-static conditions. ^^ Here, again, we follow the 

 suggestion of Gordon^* and designate the most common of these rocks as 

 granite-gneiss, diorite-gneiss, etcetera, thus clearly indicating the origin, 

 structure, and composition of the rocks. Some amphibolites and some 

 ,epidotite and chlorite schists, etcetera, also belong here. Since both 

 ortho- and para-phyllites are really intermediate between megascopically 

 non-crystalline foliates (slates), on the one hand, and gneisses and schists 

 on the other, there is some doubt as to their exact position in the classi- 

 fication. Some phyllites are probably crystalline enough to warrant 

 placing them with the gneisses and schists. 



Ortho-slate and ortho-phyllite. — These rocks are megascopically non- 

 crystalline, or only slightly crystalline, foliates which possess a more per- 

 fectly developed cleavage than the schists. They are produced essentially 

 by pressure. As pointed out above, the phyllites, which are more crystal- 

 line than the slates, are really intermediate in structure between typical 

 slates and typical schists. 



NON-FOLTATE(^ 



The non-foliated metamorphic igneous rocks are produced essentially 



3*C. H. Gordon : BuU. Geol. Soc. Am., vol. 7, p. 122. 



•""^ Adams and Barlow : Geol. Surv. Canada, Mem. 6, 1010, pp. 78-87. 



•''« W. .T. MlUer : .Jour. Geol., vol. 24, 1916, pp. 600-612. 



^ R. A. Daly : Geol. Surv. Canada, Transcontinental Guide Book 8, pt. .3, 1913, p. 130. 



8« C. H. Gordon : Bull. Geol. Soc. Am., vol. 7, p. 122. 



XXXIV— Bull. Gbol. Soc. Am., Vol. 28, 1916 



