1884.] 31 [Bouv<5. 



that were eventually consolidated into the argillite, than can be 

 ascribed to causes now in action however prolonged in duration. 



Before, however, proceeding further I wish to have borne in 

 mind the apparent fact that the area of the basin, so-called, was 

 one of great igneous activity during and following the deposi- 

 tion of the strata, as is shown by the very numerous trap dikes 

 that everywhere forced their way through the granitic rocks sur- 

 rounding it, responsive no doubt to the greater action within its 

 limits by which extensive eruptions of similar, if not identical, 

 character were raised to the surface through the conglomerates 

 and slates ; or rather, perhaps, through the materials composing 

 them and helping to their consolidation. I refer to the amygda- 

 loids, the exotic character of which seems to me to admit of no 

 question. 



The basic character of this rock has led me to infer its deriva- 

 tion from the diorite shown by Crosby's map of eastern Massa- 

 chusetts to border the basin at Waltham and Belmont, and 

 which may have originally extended over its whole area. The 

 view advanced by another geologist that the amygdaloid has been 

 derived from conglomerate is untenable, as the result of such 

 change would have been the production of an acid exotic and not 

 a basic one. 



Having thus spoken of the character of the rocks composing 

 the contents of the basin, let me present such a view of the ori- 

 gin of the great accumulation of sedimentary deposits as ap- 

 pears to me probable. To fully realize all that may be expressed 

 on this point it becomes necessary to consider the unquestionable 

 condition of the whole land surface of the continent, prior to the 

 great ice period during which by glacial action all the superficial 

 materials of the northern portions were disturbed, inextricably 

 mixed, and borne along away from their original resting places to 

 form the great body of the drift. 



What was the character of the material making up the whole 

 surface of the land previous to the deposit of the strata of 

 the Boston basin ? No one who has studied the subject at all 

 will doubt that it was composed of the wholly or partially disin- 

 tergrated remains of early formations caused by a process of 

 decay that had been going on for countless ages, facilitated no 

 doubt by an atmosphere more favorable for such action than 

 that of the present period. 



