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and Chestnut Hill band of slate, says : " The total thickness of 

 the formation," referring evidently only to the conglomerate, 

 " remains yet a matter of question ; but it cannot be less than 

 twelve hundred to two thousand feet, and may be twice the latter 

 amount." While in the very next paragraph, relating to the 

 slates of Cambridge and Somerville, which have an apparent 

 volume, as just stated, fully equal to that of the conglomerate, 

 — lying at an average angle of about twenty-five degrees 

 for a horizontal distance across the strike of two miles or 

 more, — we find the following : "The aggregate thickness dis- 

 closed in the Cambridge and Somerville sections is not far from 

 two hundred feet." Prof. Shaler has evidently regarded the 

 slates as faulted, and yet the proof is not clearer than in tbe 

 Qase of the conglomerate. 



In the light of my present knowledge I would assign the 

 conglomerate a maximum volume not exceeding one thousand 

 feet ; and consider that the greatest thickness of the slate can- 

 not be much less than that of the conglomerate, though in some 

 cases certainly falling below five hundred feet. The extensive 

 variations in lithological character and volume of these rocks 

 is another proof that they were deposited in an irregular and 

 limited basin, such as they now occupy. 



Relations of the Slate and Conglomerate to the Crystal- 

 lines. — Under this head I can only repeat, with, if possible, 

 added emphasis, the general conclusion stated in the introduction. 

 Nowhere in the Boston basin do we find a vestige of indisputable 

 evidence pointing to the conclusion that there are crystalline rocks 

 of Paleozoic age in this part of the State. Wherever the rela- 

 tions of the two great series, the crystallines and the uncrys- 

 tallines (including the Shawmut group with the former) , can be 

 clearly observed, their entire unconformability is perfectly evi- 

 dent. Every crystalline rock in this region is represented in the 

 pebbles of the conglomerate, and this rock certainly underlies 

 the slate. 



Only in a few limited localities, where the slate and con- 

 glomerate have been altered by contact wdth intrusives, — as at 



