EXCAVATIONS. 235 



CHAPTER IX. 



Embracing excavations or denudations, valleys ; lakes ; scratches upon rocks which 

 have been smoothed ; and mounds. 



Excavations, Valleys. 



No part of the Union presents such obvious and extensive excavations as New-York, and 

 it is owing to this circumstance that such facihties are offered for the study of its rock masses. 

 Reversing the order in which the excavations or denudations have been made as to time, the 

 first, being the lowest depression of the district, is Lake Ontario. The accordance of the 

 rocks on both its sides, and the dip of the rocks, are aUogether favorable to its being a lake 

 of excavation. No other cause could have given rise to it, but the sinking of the mass which 

 forms its area ; a supposition which is not in harmony with the nature of its outlet, and dip 

 of its rocks on all its sides ; the parts of the whole being in perfect conformity to each other, 

 and in accordance with a lake of excavation. 



The next and more extensive excavation or denudation, is the Mohawk valley and the 

 Great level ; these comprehend all that part which extends north from the Helderberg range, 

 to where it thinned out, or originally terminated, and which is as yet wholly unknown. As 

 the Marcellus shales and the Hamilton group rise in hills, and in some points quite near the 

 edge where the range commences, it is certain that from the top of the range to the bottom of 

 the valley of the Mohawk river, including the Great level, was not the whole height from 

 where the excavation commenced, but that it ascended yet higher. 



The third and last series of excavations, and the highest and first that were made, are the 

 north and south parallel valleys and their lakes to the south of the Helderberg range ; they 

 are coextensive with the district, and disposed over its surface at intervals of a few miles. 



The assertion that these valleys were first excavated, is founded upon the fact, that in 

 every one of them, and over their hills, the larger portion of their rolled stones are of northern 

 origin, consisting of primary rock, grey and red sandstone ; the latter sometimes showing its 

 Fucoides harlani, and amongst them occasionally some of the harder varieties of Pulaski 

 sandstone with its peculiar fossils. These stones are in such prodigious number, that their 

 existence where seen can only satisfactorily be accounted for by the extension of the rocks 

 north, which, by the dip of the rocks, would gradually bring them upon the same plane ; just 



