Johnson and Warren — Geology of Rhode Island. 5 



shows as a dark resinous, crystalline, splintery and compact 

 mass, holding porphyritically enclosed feldspar. This, in the 

 altered forms, passes into a rock, having the same groundmass 

 but with the feldspar altered into patches of a dark green, fine- 

 grained, serpentine product. By further alteration the rock 

 changes to a dark grey, serpentinous rock, spotted with the 

 decomposition products of iron. In the altered forms short 

 brilliant crystals of actinolite are to be seen and the rock often 

 shows a schistose structure." 



Again in 1893 he again refers to the cumberlandite in one 

 of his reports to the Michigan Geological Survey and gives 

 two analyses by Professor R. L. Packard to show the general 

 composition of the rock. These analyses are the best analyses 

 of the rock heretofore made, and appear to be essentially cor- 

 rect except for certain omissions, notably the alkalis. A 

 partial analysis of a very impure feldspar separated from the 

 rock is also given. No statement as to the true composition 

 of feldspar present in the rock appears in any of the papers 

 cited, and as no attempt was made, so far as known, to determine 

 sodium, it is inferred that the feldspar w T as generally supposed 

 to be anorthite. 



In the same report he states that, " the form containing feld- 

 spar was found by field observation in 1885 to pass into the 

 form containing only greenish spots of serpentine and am phi- 

 bole. Feldspars were then found showing various gradations 

 in the alteration to the serpentinous products." In comparing 

 the altered types with the feldspathic, the relation existing 

 between the feldspar phenocrysts and the green spots is so 

 obvious that it is a little surprising that so important a fact 

 was not pointed out before. 



In 1883, Professor JN. S. Shaler published a paper on "The 

 conditions of erosion beneath the deep glaciers, based upon 

 a study of the bowlder train from Iron Hill, Cumberland, 

 R. I." In the course of the paper he devotes considerable 

 space to a description of the hill. He states that the dipping 

 magnetic needle indicates that the rock mass has an extent 

 about equal to the one assigned to it from surface showings, 

 and that there is no evidence that similar masses exist, at least 

 within an area of many miles about the hill. After a brief 

 review of the facts in hand, he concludes that the cumberland- 

 ite rock mass is dike-like in its mode of occurrence and that it is 

 an igneous intrusion. His discussion of the glacial phenomena 

 about the hill and of the remarkable bowlder train from the hill 

 traced as far south as the island of Martha's Vineyard, is 

 exhaustive. He estimates from his observations that probably 

 300 feet in vertical thickness have been removed from the rock 

 mass by the glacial erosion, assuming that the original area 



