1884.] 33 [Bouv4. 



suffered through chemical action from the decomposition that 

 affected the mass, so that there maybe always found boulders and 

 pebbles of all sizes unchanged, showing distinctly the original 

 character of the whole. This was probably due to a gradually 

 lessening intensity of corrosive action and finally to its almost 

 entire cessation. 



Let us now picture the probable condition of things in and 

 about the Boston basin prior to events which led to its formation. 

 I presume there can be no question but that the country was undu- 

 lating, with hills of much higher altitude than now, and that the 

 basin itself was a valley extending farther east than at the pres- 

 ent day, the land surface reaching continuously beyond where now 

 rest the islands of the harbor. There is too good reason for 

 the belief that then as now rivers flowed through it to the ocean. 



Now, if it is admitted, what no geologist will I think ques- 

 tion, that the substance of the surrounding hills and valleys 

 was composed of the disintegrated and partially disintegrated 

 rock formations preexisting in the same localities and that there 

 yet remained disseminated throughout and composing a large 

 portion of the mass, boulders and pebbles not decomposed, we 

 have with the exception of such as was of igneous origin all the 

 material required for the production of the great deposits under 

 consideration. 



I wish that what has been said relative to the undecomposed 

 boulders and pebbles remaining in the otherwise disintegrated 

 rock formations of early periods be especially borne in mind, 

 as I believe the production of the conglomerate of the Boston 

 basin, as well as that of many other regions, to have been due 

 mainly to the deposit of this unchanged material where subse- 

 quent influences were favorable to its consolidation. 



All circumstances conspire to show that at a period anterior to 

 the formation of the basin there commenced a series of the vio- 

 lent igneous disturbances before referred to as having occurred 

 in the region, and which finally led to the more or less gradual 

 sinking of the whole area of the basin until at length the depres- 

 sion reached a depth of nearly or quite two thousand feet. Such 

 depression would of course tend to increased igneous activity 

 beneath, and further eruptions of the underlying melted matter 

 would be likely to come to the surface. This being the case it 



PROCEEDINGS B. S. N. H. VOL. XXIII. 3 NOVEMBER, 1884. 



