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NEW YORK STATE MUSEUM 



bedded and like an integral part of the series. But their com- 

 position and character seem to point to the igneous origin of a 

 large part, if not the whole, and they likely represent dikes, 

 sheets and small intrusive masses, somewhat later than the 

 sediments. 



All these rocks are often cut by rusty looking amphibolite 

 dikes which cut across the foliation, and represent somewhat 

 later intrusions, likely of diabase, but so old and profoundly 

 altered as to present little resemblance to the original rock 

 except in composition. 



These gneisses are further cut by yet later granites and gab- 

 bros, rocks which have not been so profoundly metamorphosed, 

 but which still retain sufficient traces of their original structures 

 as to render their origin certain. The finding of these is quite 

 what should be expected, since all younger igneous rocks must 

 have cut their way through these gneisses in working their way 

 toward the surface. 



At the present day the Grenville rocks are found in numerous 

 long, narrow belts around the borders of the Adirondacks, but 

 mostly only in small, disconnected patches in the heart of the 

 region. The reasons for this will appear later, but in a word it 

 seems due in large measure to the greater amount of erosion 

 which the Precambric rocks have undergone in the latter situa- 

 tion. The belts are larger and more numerous on the south and 

 west sides, and are infrequent on the north, except at the extreme 

 northwest. Kemp has emphasized the greater abundance of lime- 

 stone in the belts on the west and the greater quantity of quartzose 

 gneisses on the east. In the large way this is the case, though 

 both rocks are found in each locality. Both are also found in 

 the scattered patches which extend through the heart of the 

 region. 



The base and summit of the Grenville are both unknown, and 

 the thickness of the series is therefore purely a matter of guess- 

 work. The rocks were deposited all over the region, but erosion 

 has removed all but the present belts and patches. The lack of 



