152 Scientific Geology. 



This deluge, as I have already stated, swept over Massachusetts 

 from the north and northwest. The proof of this position I now 

 proceed to exhibit. 



The first part of this evidence consists in tracing- erratic bowlders 

 to the parent rock from which they were derived. 



When I began an examination of the State, I traveled east and 

 west ; commencing with the line of towns bordering upon Con- 

 necticut, and returning through the line of towns next north. Thus 

 essentially have I gone over the whole State. And I had not thus 

 doubled my course many times, before I found, uniformly, that in 

 order to trace bowlders to their original beds, I must travel north a 

 greater or less distance. The discovery was frequently of great ser- 

 vice to me ; and I do not recollect that the principle ever failed me. 

 I have, indeed, sometimes found a straggling block east or west, and 

 possibly north of ledges of the same kind of rock ; but never any 

 thing more than lonely stragglers. It will be expected, however, 

 that on such a point I should refer to particular instances. 



I have already remarked that granite and sienite constitute th e 

 great mass of the bowlders scattered over the southeast part of the 

 State ; and that these correspond to the rocks of this character on the 

 coast that bounds Boston harbor. But similar rocks also occur in 

 place, occasionally, in the region where the bowlders are found ; 

 and, therefore, we cannot be sure that they were transported from a 

 distance ; although in many cases the exact correspondence between 

 the specimens would leave little room to doubt that such was the 

 fact. But scattered among these primary bowlders, we frequently 

 find others of porphyry, compact feldspar, and gray wacke conglom- 

 erate ; rocks, which (except the conglomerate,) occur only within a 

 few miles of Boston, both north and south. I have found masses of 

 porphyry as far down Cape Cod as Orleans ; and near the southern 

 extremity of Martha's Vineyard, the pebbles of this rock are quite 

 numerous. In Tisbury I have seen a mass of peculiar blood-red, 

 compact feldspar, which occurs in place in Hingham : which would 

 indicate the course of the diluvial current to be a few degrees east of 

 south. The porphyry pebbles merely indicate a southern direction 

 to the current ; since the occurrence of porphyry at Half way Rock, 

 east of Marblehead, shows that this rock might formerly have ex- 

 tended far into the ocean. Gray wacke conglomerate occurs in the 

 the gray wacke formation in patches, from Rhode Island to New- 

 buryport ; and the bowlders of it above spoken of, must, therefore, 



