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feet in diameter are common, and I have observed those which 

 were four, and even six, feet in the largest dimension. Passing 

 upward, this gives way to the more normal and common type of 

 conglomerate, — the true pudding-stone, in which the pebbles 

 rarely exceed six inches in diameter, and are well rounded. Still 

 higher, limited beds and patches of grit and sandstone are inter- 

 calated, as already described ; and these appear more frequently 

 as the pebbles become smaller, finally including layers of slate ; 

 and thus the complexion of the formation is gradually changed 

 from a coarse breccia to an impalpable and homogeneous argil- 

 lite. From the earliest conglomerate to the latest slate, the 

 deposition has evidently been substantially uninterrupted, and 

 has gone on during a period of progressive subsidence with 

 which the growth of the deposits did not keep pace. In map- 

 ping these rocks, my rule has been to draw the boundaries so 

 as to include all the grit and sandstone with the conglomerate. 



The suggestion advanced by some writers, that the conglom- 

 erate may be an ancient glacial deposit analagous to our mod- 

 ern drift, a Paleozoic boulder-clay, seems to be disposed of by 

 the lithologic passage just described, which certainly exists as 

 a general fact, though perhaps not observable at every exposure 

 of the contact ; and by the undoubted conformability of the con- 

 glomerate and slate, and the very evident stratification of much 

 of the former, together with the entire absence, so far as known, 

 of striated pebbles, and the extremely local origin of the materi- 

 als. The general absence, in the conglomerate, of far-travelled 

 materials, except perhaps in the case of some of the quartzite 

 pebbles, is very noticeable. The pebbles of the conglomerate, 

 too, form a larger proportion of the rock, are generally smaller, 

 and of more uniform size in the same part of the rock (showing 

 the sorting power of water) , than is usual with the modern 

 till. There are, probably, also, few physical geologists who 

 would now admit the possible existence of glacial conditions in 

 Paleozoic times. 



In what precedes I do not mean to assert that all the con- 

 glomerate is strictly inferior to, i. e., older than, all the slate ; 



