AMERICAN GEOLOGY — DECADE OF 1850-1859. 47 1 



We have no doubt that a part of what we call drift phenomena in New England 

 was produced by glaciers such as we have described as once connected with the 

 Green Mountain range. But the main features of drift we impute to icebergs and 

 ice floes as the continent was gradually sinking beneath the ocean. 



He thought, too, to have found evidence that the continent went 

 down to a depth of .something like 2,000 feet, and as some of the ice- 

 bergs in the Atlantic have been estimated as extending 2,500 feet 

 below the surface, he argued that the effects of such could not be dis- 

 tinguished from those of glaciers. Indeed, the idea of a continental 

 ice sheet was still beyond his comprehension, and his chief arguments 

 against the glacial origin of much of the drift was based upon the fact 

 that all known glaciers are confined to valleys; and, moreover, that 

 there was no known glacier more than 50 or 60 miles wide, "whereas 

 the ancient American glacier must have been at least 2,500 miles wide 

 and have spread over all the mountains as well as valle} T s, and often 

 have been obliged to move uphill as well as over a level surface/' 

 The three prominent directions of the drift were to him also arguments 

 against the ice sheet. These were essentially the ideas advanced in 

 his Surface Geology (p. 402). 



On lithological grounds the alluvial deposits were classified as (1) 

 drift and (2) modified drift, and on chronological grounds into four 

 periods, in each of which the continent was differently situated in 

 respect to the level of the ocean. These were, first, the drift period, 

 when the continent was under water at its greatest depth; second, the 

 beach period, when it began to emerge; third, the terrace period, 

 when the continent rose to nearly its present situation; and, fourth, 

 the historic or present period. The phenomena of the modified drift 

 as terraces and beaches were worked out in detail in this report by 

 C. H. Hitchcock. 



In the chapter on hypozoic and paleozoic rocks Hitchcock expressed 

 the belief that the rocks of Vermont have been thrown into a succes- 

 sion of folds while in a semiplastic condition by a force from the 

 direction of the Atlantic, and that their crests have been subsequently 

 denuded, some 10,000 vertical feet having, it was estimated, disap- 

 peared from the surface of the Shelburne anticline. The strata of 

 those folds with westerly dips, on the western side of the anticlines, 

 he regarded as occupying a normal position; while those with easterly 

 dips, on the eastern side, had been inverted, so that, as he expressed 

 it, "though we cross an uninterrupted succession of easterly dips in 

 going eastward we can not infer that we are constantly meeting with 

 older and older rocks, and therefore that mere superposition would 

 not justify one in deciding upon their relative ages." 



Concerning the position of the much-disputed red-sand rock, Hitch- 

 cock remarked that without an exception it rested upon the Hudson 

 River group, and the stratigraphical evidence showed it to be of the 



