10 



lines of the subjacent rocks than we are able to detect in the 

 existing hills and valleys considered as a whole." One of the 

 most remarkable facts in the distribution of glacial detritus, or 

 drift, in Massachusetts, as I have already observed, is the com- 

 paratively great depth to which it has been accumulated over 

 the south-eastern portion of the State. There is a marked 

 paucity of rock outcrops in the southern half of Plymouth 

 County ; south of Plymouth and east of Middleborough they 

 are rarely met with ; and Barnstable County is absolutely des- 

 titute of them. It is not improbable that the solid rocks in 

 this region are so deeply buried by the unconsolidated superficial 

 deposits, that, if the latter were removed, the whole of Barn- 

 stable County and a considerable part of Plymouth County 

 would be invaded and covered by the sea. Certainly here, if 

 anywhere, we may expect lake basins and river valleys to ex- 

 hibit in their forms and trends a complete independence of the 

 underlying rocks ; and, as I have shown in the paper above 

 cited, this expectation is fully justified by the facts. In 

 Worcester County, and north of this region of excessive drift, 

 however, the dependence of the surface lineaments on the geo- 

 logic structure is very marked. This is especially noticeable in 

 the case of the larger features, such as the Blackstone, Nashua, 

 Merrimac, Parker, Ipswich, Charles, Neponset, and other 

 rivers ; the Wachusett range of highlands ; the parallel range 

 forming the eastern rim of the Nashua valley ; the somewhat 

 irregular belt of hills extending from Cape Ann to Beverly ; the 

 well-known range sweeping with bold front from Swamp scott 

 to Waltham ; and the Blue Hill range in Milton and Quincy. 

 The last three lines of hills, being principally composed of un- 

 stratified rocks, are less regular and distinct than the others ; 

 yet they no less clearly reveal the structure of the rocks com- 

 posing them ; for exotic rocks, being, in a certain sense, struc- 

 tureless, only conform with the general law in giving rise to 

 a systemless topography. 



The rocks of Eastern Massachusetts admit of a convenient, 

 and, it is believed, a chronologic, division into two great 



