462 NEW YORK STATE MUSEUM 



since in composition they are close to some igneous rocks. But 

 their banded character, and the occasional appearance of thin bands 

 which are quite certainly sedimentary, strongly suggest a sedi- 

 mentary character for the whole. Such gneisses occur in force in 

 the Moose creek belt, along Bog river. The garnetiferous gneisses, 

 both acid and basic, which usually play such a large role in the 

 Grenville make little show in the district, though occurring in 

 small amount , here- and there. 



The Grenville rocks are all cut up by other rocks -which seem 

 igneous. Some of these are plainly to be classified with the later 

 great intrusions; but others are quite unlike these. Of these last 

 some resemble phases of the gneisses here classed as doubtful, and 

 shortly to be described, while yet others seem to be peculiar to the 

 Grenville association and not to be found elsewhere. Yet their 

 discrimination is a matter of great difficulty, there are so many 

 phases of the other rocks to be borne in mind. Further their sup- 

 posed diagnostic characters are much easier to recognize than to 

 describe. Some rocks which seem quite certainly igneous are often 

 apparently interbanded with the sediments, and may represent 

 heavily metamorphosed contemporaneous sheets, or flows, or even 

 beds of volcanic ash. Other rocks which are unquestionably igneous, 

 cut the sediments, and yet have not so far been certainly recognized 

 away from the Grenville association. While indisputably later 

 than the rocks which they cut, they are thought to be not greatly 

 younger, and to be much older than the big intrusions. Yet the 

 whole question is an exceedingly difficult one, and the poor and 

 sparse rock exposures of the Grenville throw little light upon it. 

 Some of these rocks have the composition, or at least the mineral- 

 ogy, of granites, some of syenites, and some of gabbros. The latter 

 are perhaps more apt to be distinctly interbedded with the sedi- 

 ments than are the others, though all seem to have that occurrence 

 at times. 



There seems considerable uniformity of structure in the different 

 Grenville belts. The general strike varies from west to northwest, 

 and the usual dips are to the south and frequently high. In the 

 Rock pond belt the strike varies between n. 30 w. and n. 6o° w. in 

 the Grampus lake vicinity, and from n. 6o° w. to west nearer Long 

 lake, hence has general parallelism with the trend of the belt. In the 

 Moose creek belt the exposures are very poor and the dips are flat, so 

 that it is difficult to get observations of any pretense to accuracy upon 



