294 NEW YORK STATE MUSEUM 



sea to run up the valley into Lake Ontario and to invade the Cham- 

 plain valley also, producing a huge, branching estuary. In these 

 waters marine sands and clays were deposited, and, though un- 

 consolidated, these still remain in considerable bulk, their marine 

 origin distinctly shown by the marine fossils which fhey contain. 



Gradual elevation of the region since, greatest at the northeast, 

 and amounting to about 600 feet at the lower end of Lake Cham- 

 plain, has brought the region above sea level and shrunk the 

 St Lawrence estuary to its modern proportions. This upward 

 movement is probably still in progress. 



But a few thousand years have passed since the ice disappeared. 



Erosion has made but little progress in obliterating its traces 



except along the immediate stream valleys. Since the streams 



have been more or less shifted from their preglacial courses, 



they have been obliged to recarve their valleys in whole or in part, 



and they are actively at work at this task. The steady rise of 



the land in postglacial times has given them a steadily lowering 



base level, and, though they have cleared away much of the glacial 



deposit from their paths, the amount of rock cutting done is not 



great. 



THE EOCKS 



, Precambric rocks 



While in many parts of the Adirondacks areas of varying size 

 are found in which the rocks that occur may be unhesitatingly 

 classed as Grenville sediments or as later igneous intrusions, over 

 much of the district this is- not the case, but an intimate admix- 

 ture of various rocks is found, in apparently hopeless confusion. 

 Thus we find Grenville sediments elaborately interbanded with 

 other rocks, apparently igneous, yet seemingly conformable with 

 them as an integral part of the series. We also find rocks which 

 are not to be distinguished in appearance or in composition from 

 the rocks of the great intrusions, except for perhaps a more 

 thoroughly gneissoid charadter, and yet so interwoven with other 

 rocks, so far as yet known not repreisented in the great intrusions, 

 that it hardly seems possible that the two can belong together. 

 There are also considerable areas of gneisses which are quite like 



