Crosby.] 494 [March 7, 



any eruptive rock ; and it seems necessary to regard them as mainly, 

 if not wholly, intrusive in the slates and schists. 



The Metamorphic Conglomerate. 



The metamorphic conglomerate and the closely associated ar- 

 gillites, schists and quartzites are, except in their highly ferrugi- 

 nous character, strikingly similar to the metamorphic sediments 

 occurring in Bellingham, Massachusetts. 1 I may, therefore, for 

 the sake of brevity, refer to the published descriptions of the Bel- 

 lingham rocks for the structural details of the Black Hills series. 

 In both localities it is perfectly plain : (1) That the metamor- 

 phic series consisted originally of interstratified beds of ordi- 

 nary mechanical sediments — slate, sandstone and conglomerate; 

 (2) that the metamorphic agents have not only accomplished the 

 important chemical change implied in the fact that these rocks 

 are now in the main, distinctly hydromicaceous, but also a very 

 marked mechanical change, which is most apparent in the deforma- 

 tion of the pebbles of the conglomerate. The original slates have 

 become fissile argillites or phyllites ; the argillaceous sandstones 

 are now mainly lrydromica schists ; and the coarser sandstones, 

 or conglomerates, approximate to the same metamorphic type, 

 being highly micaceous and, through the elongation of the peb- 

 bles, often distinctly schistose ; while the pure siliceous sandstones 

 have become quartzites. 



The fragmental nature of the conglomerate and the deforma- 

 tion of the pebbles are, in both regions, indisputable facts ; and 

 in the Black Hills, as in Bellingham, the deformation is essentially 

 or mainly cylindrical, each pebble being elongated or drawn out 

 rather than flattened. The cross section of the structure shows 

 usually the appearance of a normal conglomerate ; while in the 

 side or longitudinal view this is almost entirely wanting, and we 

 see only a regular lamination or schistosity. An occasional peb- 

 ble has, however, successfully resisted the deformation ; and the 

 adjacent pebbles and the micaceous cement are wrapped around 

 it in a very instructive manner. The degree of metamorphism is 

 not uniform, but I have observed in the same mass, or group of 

 ledges, a gradation from a puddingstone showing but little altera- 



jProc. Boston Soc. Nat. Hist., XX, pp. 373-378. Occas. papers Boston Soc. Nat. Hist. 

 Ill, pp. 145-153. 



