ABSTRACTS 



97 



to thirty feet of granitic gravels and sands, the deposition of which 

 was brought to a close by profound folding over Marthas Vineyard 

 and Block Island. The Tisbury beds on Marthas Vineyard and the 

 Mohegan Bluff beds on Block Island were now laid down upon the 

 eroded surface of the upturned beds of older strata. These horizontal 

 beds are evidently of glacial origin. After their deposition there came 

 a long period of fluvial erosion, the Vineyard subepoch, in which the 

 islands were deeply denuded, as pointed out by Shaler in 1888. On 

 xMarthas Vineyard the capture and beheadal of streams is shown in the 

 existing topography as the work of this time. Then followed the last 

 glacial epoch with the deposition of moraines and sand-plains. 



The older Pleistocene deposits denominated Weyquosque by Shaler 

 in 1888 appear to be the equivalent of the Columbia which in the 

 region of the New England Islands is divisible into an upper and 

 lower horizon, separated by unconformity of dip and erosion marking 

 the Gay Head diastrophe. The author is unable to accept the hypothe''- 

 sis of glacial thrust for the deformation on the islands named because 

 in each case the fold preceded the evidence of glaciation found in 

 overlying bowlder deposits. Those who accept the ice-thrust hypothe- 

 sis, it is insisted, must consider the work to have been done' by two 

 advances of an ice-sheet long anterior to the last glacial epoch. 



In the discussion Professor Wm. B. Clark called attention to the 

 evidence of unconformity found by him in New Jersey between the 

 non-marine or Potomac beds and the marine Cretaceous, and also to 

 the meagerness of the horizons in this northern extension of the coast- 

 plain as compared with more southern fields. He suggested that an 

 examination of the foraminiferal greensand might show that, as in New 

 Jersey, the Miocene foraminiferal casts had at least in part been 

 derived from preexisting Cretaceous beds. Mr. Cxilbert noted that the 

 long erosion interval on the islands, preceding the last glacial epoch, 

 wa& consistent with the history made out for the rest of the United 

 States. 



Ho7nology of Joints and Artificial Fractures. By J. B. Woodworth. 



A synopsis of the structure of a typical joint as described in a 

 previous paper {Proc. Boston Soc. Nat. Hist., Vol. XXVII, 1896, pp. 

 163-183) showed a central area of fracture termed the joint-plane, 

 which is traversed by feather-fracture. Marginal to this is a border of 

 overlapping fractures, traversed by cross-fractures, forming the "joint- 



