Crosby.] 1 6 [January 16, 



Quincy, at Squantum, and on the north side of North Beacon 

 Street, in Brighton. The conglomerate of the last named local- 

 ity is well known, and several observers have noted the occur- 

 rence in it of the " slate pebbles." A careful study, however, 

 has satisfied me that the pebble appearance is illusory in most 

 cases at least, these masses being indigenous in their present po- 

 sitions, and not imported. In most cases the material seems far 

 too shaly and fragile for the formation of transportable pebbles 

 of the sizes observed ; the largest being a yard or more in diam- 

 eter and yet only two or three inches thick. And the rare occur- 

 rence of the so-called slate pebbles, except along particular planes 

 in the rock, where they are numerous, is decidedly a suspicious 

 circumstance ; while peculiarieties of form in many cases, as 

 where they envelop pebbles of other material or enclose arena- 

 ceous strings which are continuous with the general paste of the 

 conglomerate, seem to complete the proof. But the evidence is 

 still more convincing in North Quincy and Squantum. 



Pebbles of partially decomposed felsite are easily mistaken for 

 slate pebbles, and the felsitic debris is exceedingly abundant in 

 the conglomerate. Many pebbles that seem to be slate are very 

 compact and more or less altered basic eruptive rocks. Among 

 the trap and amygdaloid of the Boston basin it is common to 

 find varieties which are not readily distinguished from slate in 

 hand specimens. I do not deny the possible existence in the 

 conglomerate of genuine slate pebbles ; but I do claim that the 

 occurrence of pebbles derived from the Primordial or any more 

 recent slate formation has not been proved. It is important to 

 remember that there are many slaty rocks in eastern Massachu- 

 setts outside of the Boston basin, that some of these are almost 

 certainly of pre-Cambrian age, and that, if true slate pebbles 

 occur in the conglomerate, they may have been derived from 

 some of these older slates. 



The facts afforded by the artesian well on Providence Street 

 in this city, for which I am indebted to Mr. J. A. Whipple, are of 

 interest in this connection, since they point to the conclusion that 

 the conglomerate is below the slate. This well is 2503 feet deep. 

 Down to a depth of about 2300 feet the rock is slate, the com- 

 mon gray to black varieties, with some arenaceous beds and prob- 

 ably more or less vein quartz. Below 2300 feet the character of 



