356 Address before the Association of American Geologists. 



hundred feet below their tops, and pass over them without los- 

 ing their parallelism ; and yet the situation of the drift shows 

 that these markings were made by an ascending and not a de- 

 scending body. Such might be the effect, if the whole surface 

 of the country were covered by a thick sheet of ice expanding in 

 a southerly direction. 



Of the other case, I have met with two examples in New 

 England, and know not that they have been noticed elsewhere. 

 In these cases, the perpendicular layers of argillaceous and horn- 

 blende slate, covered in one place, by fifteen or twenty feet of 

 drift, have been fractured to the depth of ten to fifteen feet, so as 

 to be more or less separated, producing horizontal fissures, which 

 are filled by mud, while the laminas are inclined, at various angles. 

 In short, it seems as if an almost incredible force had been exer- 

 ted upon the surface in an oblique direction. Such a force might 

 be exerted by an immense mass of ice in the process of expan- 

 sion ; but I know of no other source from which it could have 

 been derived.* 



On the other hand, there are features in the phenomena of di- 

 luvial action in this country, which are explained by this theory 

 in a much less satisfactory manner. One is the southerly direc- 

 tion which our drift has taken, and the great distance to which it 

 has been carried. It cannot be conceived that any single glacier 

 should have expanded several hundred miles in a southerly direc- 

 tion, especially over a surface which could have had scarcely any 

 southerly slope. Even if we admit a Mer de Glace in the north- 

 ern regions so lofty as, in the beginning of the work, to send gla- 

 ciers a vast distance, yet the force seems to have continued to 

 operate in the same austral direction, even to the bottom of our 

 valleys. It is, however, probably true, that the great mass of our 

 drift will be found within fifteen or twenty miles from its original 

 place ; and that which occurs at greater distances, may perhaps, 

 have been transported by powerful currents of water. It is al- 

 most certain that the sheet of ice which covered the surface, ac- 

 cording to this theory, must have been at least three thousand or 

 four thousand feet thick, because our mountains have been to that 

 height, swept over. Now if, as Agassiz and others suppose, 

 the fall of temperature, at the beginning of the glacial period, was 



* Descriptions and sketches of these cases are given in the Final Report on the 

 Geology of MassachusettKS, Vol. ii, p. 396 and 559. 



