1869.] 175 [Shaler. 



hills across the Braintree rocks in a northeasterly direction, the total 

 thickness of unquestionably stratified beds is not far from one thou- 

 sand feet. The uniform dip away from the Quincy Hills, shown by 

 all the stratified beds on their flanks, may be regarded as sufficient 

 proof that their elevation came after the deposition of these beds. 



The region about the Quincy Hills does not afford any sufficient, 

 evidence concerning the section above the line of the Braintree series. 

 At Weymouth Landing there is a great mass of schistose rocks which 

 certainly are not far removed from this part of the section, but I have 

 not yet succeeded in tracing any connection Avith it. I am inclined 

 to think that they may be the beds between the Braintree slates and 

 the quartzites before described. 



In addition to the rocks already mentioned, there exist two other 

 sets of beds, which are extremely developed in the environs of 

 Boston, the relations of which are not easily determined. These 

 are the series of tbe Roxbury conglomerate, and the set of beds 

 which I have chosen to term from the point where they are seen to 

 the best advantage, the Cambridge slates. The first of these consists 

 of a mass of stratified pebbles, with intercalated sandstones and 

 grits, the whole capped by a series of ripple marked sandstones about 

 one hundred feet in thickness, and at certain points overlaid by 

 masses of amygdaloidal trap. The total thickness of the forma- 

 tion 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. The whole is evidently a shallow water and shore de- 

 posit, being formed in a sea which became progressively more and 

 more shallow, the uppermost beds affording evidence that they were 

 deposited very near the shore. 



The Cambridge slates show by their structure that they, like the 

 Braintree slates, were deposited in deep water. In general character 

 they resemble the latter rocks, being of dense argillaceous material, 

 with little admixture of foreign substances, and presenting numerous 

 planes of jointing, and one distinct cleavage along the line of strati- 

 fication. The aggregate thickness disclosed in the Cambridge and 

 Somerville sections is not far from two hundred feet; though owing 

 to the fact that neither the base nor summit of the beds is observ- 

 able, it is impossible to determine their depth. The only clear 

 evidence of organic life consists in numerous but indistinct impres- 

 sions of fucoids. A number of other peculiar forms have been found, 

 but nothing satisfactory has yet been ascertained concerning them. 



