92 THEORIES OF DRIFT. 



land had risen much. Hence the denudation of the surface and the accumulation of the 

 unmodified drift must have been an earlier work. It was not indeed completed during 

 the subsidence of the land, nor is it yet completed. But the period of subsidence seems 

 to have been the time when the work was carried on with the greatest energy, 



We regard the drift agency, then, to have been mainly icebergs, icefloes, and shore ice, 

 while the country was sinking beneath an ocean made arctic by currents from an arctic 

 sea a state of things mainly such as now exists in high latitudes. To these were added 

 the effects of glaciers, which under such circumstances must have existed. Mountain 

 slides, waves of translation, and the ice of rivers, might also have aided in the work. 



Now the same agencies continued to operate, modified indeed and with less intensity, 

 while the continent was rising out of the waters ; and we find the results in what we call 

 Modified Drift. For we suppose that the ice and water would now act mainly upon the 

 drift already accumulated, reducing it into finer portions, sorting the materials according 

 to the strength of the current, and re-depositing them as stratified masses in the form of 

 Beaches, Sea Bottoms, and Terraces. These all lie above the drift, and must have resulted 

 from 'subsequent operations. It is impossible, however, to say where drift proper ends 

 and modified drift begins. Indeed, after the latter in some cases had been accumulated 

 to considerable depth in the form of sand and gravel, there is evidence, in the large bowl- 

 ders lying occasionally on their surface, that icebergs continued their work of transporta- 

 tion. Yet gradually the erosive agents became less powerful, and the materials deposited 

 finer, until in our day the almost impalpable powder of alluvial meadows shows us how 

 altered and diminished is the drift agency ; though existing glaciers make a much nearer 

 approach to their ancient power. 



We adopt the theory, then, that the gradual drainage of the country as it rose out of the 

 ocean by almost imperceptible increments, or as the waters retired while the continent 

 stood still, which hypothesis has certainly strong arguments in its favor. At first, only 

 a few of the highest mountains would project, as islands above the waters. But the 

 waves and currents would begin their action upon the shores and the bottom, and the 

 materials worn off and comminuted would be arranged in the form of beaches or sea- 

 bottoms. But not till a considerable part of the land had emerged, and chains of lakes 

 were produced, and incipient rivers formed currents through them, would regular terraces 

 begin to be deposited. But the modes in which these and other forms of modified drift 

 have been produced, will require numerous details in order to be made intelligible, and a,s 

 it is a branch of geology comparatively new, we allow one of our number, who has made 

 this department of the geology of Vermont an object of special study, to occupy a good 

 deal of space in describing the phenomena as they occur in the State, and in drawing 

 general conclusions. It is a branch of the survey, however, that has been more imper- 

 fectly attended to than any other perhaps, as none of us have felt authorized, with the 

 limited means at our command, to give much attention to surface geology, except in 

 connection with the study of the older rocks. Yet we cannot doubt that the facts will 

 awaken a deep interest in every one who sees what remarkable changes they indicate in 

 the surface. 





