Wads worth.] 130 [November 1, 



The subsidence of two thousand feet, of which El Yunque is a 

 monument, must have reduced the Greater Antilles to a few lines 

 of small, but high and ragged, islands ; and, as Mr. Bland has 

 shown, fully accounts for the absence in these immense tracts of 

 all large land animals, although they were abundant here in 

 Pliocene and earlier times. 



THE ARGILLITE AND CONGLOMERATE OF THE BOSTON 



BASIN. 



BY M. E. WADSWOETH. 



In a paper published in the Harvard University Bulletin (1882, 

 ii, 431, 432) " On the Kelations of the So-called Felsite to the Con- 

 glomerate on Central Avenue, Milton, Mass." it w T as pointed out 

 by the present writer that the supposed felsite of that locality was 

 simply a portion of the conglomerate somewhat altered by water 

 action (thermal) and therefore the conclusions that were drawn 

 from that locality by Mr. W. O. Crosby needed to be revised. 1 



The question of the relation of the argillite to the conglom- 

 erate of the Boston basin was briefly touched upon, hence it is 

 advisable here to call attention to some additional evidence bear- 

 ing on those relations, derived from various localities in this 

 basin. 



The best locality for observing the junction of the argillite and 

 conglomerate that I have thus far seen is on the north side of 

 Beacon street, Newton Center, near the stream running from 

 Hammond's Pond. This locality is on the land of Hon. R. R. 

 Bishop, and by the side of the street. 



The argillite is very fine grained and continues the same in 

 character up to the conglomerate. The lamination is well marked 

 and frequently contorted. At the junction the argillite is found 

 to have been irregularly eroded, and the conglomerate laid down 

 unconformably over it. In places the layers of argillite are cut 

 off and the conglomerate abuts unconformably against and over 



i Amer. Journ. Sci. 1880 (3) xix, 166-122; Occas. Papers Bost. Soc. Nat. Hist, 

 1880, in, 89, 220, 221, 270, 271. 



