ORIGIN AND RELATIONS OF GRENVILLE-HASTINGS SERIES. 399 



In petrographical character the Fundamental gneiss is more or less monotonous. 

 It consists chiefly of orthoclase gneisses of various kinds, with which are associated 

 dark basic masses of amphibolite and pyroxene granulite. The orthoclase gneisses 

 have the appearance of igneous masses in which a more or less distinct foliation 

 has been induced by movements. The associated basic masses are very dark or 

 black in color. They are usually indistinctly foliated, but are sometimes quite 

 massive, occurring in bodies of all sizes and shapes scattered through the gneiss, 

 and in the great majority of cases so intimately associated with the latter that it is 

 impossible to separate the two in mapping. 



FUNDAMENTAL GNEISS 



There can be but very little doubt that this orthoclase gneiss of the Fundamental 

 series is a truly igneous rock. There is no evidence of its having ever formed part 

 of a sedimentary series. The origin of the basic members of the series is as yet 

 uncertain. Similar basic rocks are found the world over in the Archean gneisses, 

 and they are probably closely related in origin to the pyroxene granulites of the 

 Saxon Granulitgebirge. They are probably either differentiation products from 

 the original magma or basic intrusions, whose structural relations and character 

 have been largely masked by the great movements which have taken place in the 

 whole series at a later date. 



GRENYILLE SERIES 



The Grenville series differs from the Fundamental gneiss in that it contains cer- 

 tain rocks whose composition and minute structure mark them as highly altered 

 sediments. These rocks are in part limestones and in part peculiar gneisses, rich 

 in sillimanite and garnet, having the composition of shales, or very rich in quartz 

 and passing into quartzites, having thus the composition of sandstone. These 

 rocks, as has been shown in one of the papers before referred to, usually occur in 

 close association with one another and are quite different in composition from any 

 igneous rocks hitherto described. These rocks it is which are considered as char- 

 acterizing the Grenville series. They usually, however, form but a very small 

 proportion of the rocky complex of the areas in which they occur and which, 

 owing to their presence, is referred to the Grenville series. They are associated 

 with and often enclosed by much greater volumes of gneisses and amphibolitic 

 rocks identical in character with those of the Fundamental gneiss. The limestones 

 are also almost invariably penetrated by great masses of coarse pegmatite, and in 

 some cases large occurrences of the limestone are found completely embedded in 

 what would otherwise be supposed to be the Fundamental gneiss. The whole thus 

 presents the character of a series of sedimentary rocks, chiefly limestones, invaded 

 by great masses of the Fundamental gneiss, and in which possibly some varieties 

 of gneisses present may owe their origin to the partial commingling of the sedi- 

 mentary material with the igneous rocks by actual fusion. There is, however, no 

 reason to believe from the evidence at present available that any considerable part 

 of the series has originated in this last mentioned manner. 



RELATIONS OF FUNDAMENTAL GNEISS AND HASTINGS AND GRENVILLE SERIES 



The relations of the two series in central Ontario, as determined by the investi- 

 gations of the last two seasons, throw new light on the problem and indicate its 



UX— Bull. Geol. Soc. Am., Vol. 8, 1896 



