EASTERN MASSACHUSETTS. 



439 



mistakes regiirdiiig the character of the rocks had been made. 



the occurrence of felsitc dikes in the granite, as had been previously 

 indicated by Dr. AVadsworth and Mr, Dillcr. (Proc. Bost. Soc. Nat. 



Hist, 1881, XXL, pp. 197-21G.) 



In 1881 Dr. AVadsworth showed that the rock which Mr. Crosby had 

 regarded as slate or sandstone on Marblehead Neck was really a lava 

 flow, — a trachyte ; and hence it ai>pearcd that the theory of the lat- 

 ter regarding the filling of Marblehead Harbor by sandstone, and its 

 subsequent erosion, was based on an error regarding the nature of the 

 rock in question. (Ibid., pp. 288-"294:.) 



Dr. AVadsworth also showed that the so-called diorite or syenite of 

 IMarblehead was a diabase. (Ibid., p. 30G.) 



It was later pointed out that a supposed felsite on Central Avenue, 

 I^Iilton, from the detritus of which the associated conglomerate was be- 

 lieved by .Mr. Crosby to have been formed, was simply a modified por- 

 tion of the conglomerate itself, and that here as elsewhere the usual 



(Bull. 



Ilarv. Univ., 1882, II., pp. 431, 432.) 



In a paper presented to the Boston Society of Natural History in 

 1883, it was pointed out by Dr. AA^adsworth that the Roxbury conglom- 

 ei-ate uuconformably overlaid the argillite, and contained fragments of 

 the latter. The argillite showed an old eroded surface, with its strata 

 cut off j while abutting against the laminae was the conglomerate that 

 had been deposited on this surface. Mr. Crosby had held that the con- 

 lomerate lay beneath the argillite, and explained the appearance at the 

 locality in question by supposing the existence of a closely folded syn- 

 clinal; but the cvidcneo that the conglomerate is really uuconformably 

 laid down on the older argillite and contains its debris, Dr. AA^adsworth 

 thinks, must be patent to any one who carefully examines the locality. 

 No faulting exists, for the two rocks are so closely adherent that both 

 can be removed us a single piece, which distinctly shows the xmconform 



ability. 



■with tlie conglomerate, and that a conglomerate of a later date than the 

 Quincy granite had been found on the northern side of that rock. The 

 argillite beneath the conglomerate in the Boston Basin is a fine-grained 

 compact rock, while that associated with the conglomerate is coarse- 

 grained arenaceous, passing into sandstone and conglomerate. To the 

 former belong the Newton, and probably the Braintree argillites; while 

 to the latter is provisionally referred the misnamed Cambridge slates. 



In 1883 Mr. AA'. AV. Dodge claimed that two granites existed in tlic 

 Quincy district, the outer being older than the inner one, but he gave 



g 



It was further shown that an argillite occurred interbeddcd 



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