Hawkins — Notes on the Geology of Rhode Island. 471 



granular in appearance, containing a small quantity of inter- 

 mixed biotite particles. These crystals are embedded in biotite. 



Then follow several minerals intergrown, as follows : — 



Actinolite ; a felt of light green blades. 



Biotite; large finely crystalline masses, free crystals in 

 cavities. 



Epidote; compact, feldspathic, with occasional grains of 

 quartz. 



Apatite ; tiny, brilliant white crystals, on the biotite in cav- 

 ities. The crystallographic forms were determined as follows 

 by use of the two-circle goniometer: — o, (0001) ; s, (1121) ; 

 y, (2021) ; x, (1011) ; r, (1012) ; z, (3031) ; a, (1010). Also a 

 pair of faces belonging probably to a first order pyramid near 

 (6-6-12-1), a form whose presence could not be fully established 

 because of lack of material, but which is new to the species. 

 ■^Letters given are those of Goldschmidt. ) 



That the limestone beds of Rhode Island were sedi- 

 mentary in origin was the conclusion of Emerson and 

 Perry (op. cit., 16), and is also the opinion favored as 

 the result of the present studies. In metamorphism all 

 traces of original bedding or of possible fossil remains 

 have apparently been destroyed, and other misleading 

 features, such as bands of color, have been introduced 

 by the same agency. There are restricted areas of mar- 

 ble, however, in the Dexter quarry especially, which 

 possess a very marked gray color, and seem to represent 

 brecciation products of certain zones in which the dark 

 material was earlier segregated. In the laboratory this 

 limestone was treated with hydrochloric acid, the insolu- 

 ble residue being gray. When this was brought to red 

 heat in a crucible the color quickly changed to a light 

 yellow, indicating the source of the dark color to be 

 carbon. This amounts locally to about half of one per 

 cent of the rock. It is also shown by the application of 

 further chemical tests to be in the form of finely divided 

 graphite. Carbon of this kind when present in limestone 

 is usually thought to have had its origin in organic 

 remains and to indicate the sedimentary nature of the 

 limestone. A study of the geology in Porto Rico has 

 shown 37 that the occurrence of lime beds in a sedimentary 

 series of volcanic tuffs (such as the green schists may 

 once have been) is very common in that country. 



37 Berkey, C. P., Annals N. Y. Acad. Sci., 26, 1-70, 1915. 



