OEIGIN OF THE MICACEOl'S SCHISTS. 



201 



as many modem slates and sandstones, the micaceous schists are seen in 

 the field to be interbedded with hornblende-schists of whose ig-neous orio-in 

 there can he little doul)t. It may be that some of the schists are altered 

 tufiFs, and that their banding- is due in part to the original sti-ntification of 

 the tuffaceous l)eds, as is the case with the greenstone-schists of the Northern 

 Com])lex, but of this there is as little positive proof as there is of a sedi- 

 mentiuy origin for auA* of the schists. 



Dr. Adams' has attempted to get some light on the origin of the gneisses 

 of the Grenville series in Ontario by comparing their composition with 

 that of slates anil granites. He calls attention to the fact that while the 

 average amount of the alkalis in granites is 7.3.") per cent, in 23 primitive 

 felates it is only 4.7 per cent, or two-thirds as great. Moreover, the slates 

 are much higher in alumina than the granites, while at the same time 

 they are lower in silica. The slates also contain more magnesia than lime, 

 whereas the granites contain more lime than magnesia. After making his 

 comparisons Adams concludes that the Grenville gneisses are more nearly 

 like the slates in composition than like the granites. Of course such a 

 comparison as this is of doubtful utility as a means of detennining the 

 origin of rocks that have sufifei-ed such a multitude of changes since their 

 deposition as have the schists under consideration. Even if it were known 

 that their composition had not suffered much change under the influences 

 of metamorphism, the comparative process could be of little aid in discov- 

 ering their origin, unless the composition of both the granites and the slates 

 which they yielded were known. ZirkeP has shown that the range of 

 composition in granites is very great. His maximum, minimum, and mean 

 figures for their various constituents are as follows: 



Range of composition iti granites. 



'A further contribution to our knowledge of the Laurentian, by F. D. Adams: Am. Jour. Sci. 

 3d series, Vol. L, 1895, p. 58. 



-F. Zirkel, Lehrbuch der Petrographie, Vol. II, 1894, p. 30. 



