ELIZABETHTOWN AND PORT HENRY QUADRANGLES 21 



Chapter j 

 GENERAL GEOLOGY 



Grenville series 



Introduction. Far the greater part of the area consists of the 

 ancient Precambric rocks, a very complex group, which, however, 

 can be deciphered into several recognizable and distinguishable com- 

 ponents. Along the shores of Lake Champlain in embayments or 

 projections extending from northeast to southwest up into the val- 

 leys between the ranges which come down to the lake, are found 

 the Paleozoic sediments, beginning with the Potsdam sandstone and 

 ending with the Utica slate. One dike of igneous rock has been 

 found, which cuts the Paleozoic strata. Some miles west of the 

 Qiamplain valley and separated from the main Paleozoic exposures, 

 one outlier of Potsdam has been discovered and there are indica- 

 tions of a second, ij^ miles west of Elizabethtown, although only 

 loose float has been seen. The Precambric rocks are usually 

 metamorphosed and are in instances much changed from their 

 original condition. The Paleozoics are not greatly recrystallized 

 and are much contrasted with the older formations. The two can 

 well be treated separately and, as here, by different writers. 



Precambric formations in general. The Precambric complex is 

 separable into an older sedimentary portion and a later igneous 

 portion. The sediments occupy far the lesser area, and must be but 

 a fragment of what was originally a widespread series, which has 

 been invaded, broken up and metamorphosed by the eruptives. In 

 fact one can only gain a comprehensive grasp of the total geology 

 by picturing a sedimentary area penetrated and overwhelmed by a 

 vast igneous outbreak. We have, however, only the deep seated 

 rocks. There is reason for believing that the overlying volcanics 

 which probably accompanied them were all worn away long before 

 the Potsdam epoch opened. With them went also undoubtedly a 

 vast amount of the ancient sediments. The grounds for this belief 

 are, the relatively small amount of the sediments now remaining; 

 the deep seated character of the eruptives ; and the need of assum- 

 ing some load now gone, beneath which these igneous rocks could 

 crystallize in their present coarseness of grain. It is also conceive- 

 able as an alternative that the igneous rocks were altogether 

 mtrusive, and that there were enough sediments over them to supply 

 the pressure and the conditions of slow cooling. We can only 

 contrast the two possibilities since no actual trace of one remains 

 more than of the other. 



