CRYSTALLOBLASTIC ORDER AND MINERALS 501 



development leading up to its present form; and we may assume 

 further that, in any given region, provided we consider only rocks 

 which were once of similar nature, this sequence of stages is repre- 

 sented, in a general way, by a graded series from the least altered 

 to the most altered specimens. Within the writer's personal 

 observation the best area illustrating such variations, in the case of 

 regional metamorphism, is the Narragansett Basin in southern 

 Rhode Island. 



STAGES OF METAMORPHISM IN SCHISTS OF THE NARRAGANSETT 



BASIN 



The Narragansett Basin is a structural basin consisting of a 

 downfolded and downfaulted block of Carboniferous mudstones, 

 sandstones, and conglomerates of fresh water deposition. These 

 strata were folded and were altered by dynamic and static meta- 

 morphism during the Appalachian Revolution. They appear to 

 have been near enough alike in their original state to justify a 

 comparison of them now. 



The characters of these rocks, as investigated by the writer,^ 

 led him to group them, according to their degree of meta- 

 morphism, in four stages, designated A, B, C, and D. ''The 

 metamorphism is incipient in Stage A; distinct, but rather low, 

 in Stage B; considerable to high in Stage C; and at a maximum 

 in Stage D."^ 



The criteria used in distinguishing between these stages were: 

 amount of granulation or distortion of clastic components; defor- 

 mation of pebbles, fossils, and such original structures as cross- 

 bedding, ripple-mark, etc.; proportion of new or metamorphic 

 minerals; proportion of recrystallized components; shape of 

 mineral grains; mode of aggregation of constituents; degree of 

 parallelism of minerals with unequal dimensions and of elongate 

 mineral aggregates; perfection of rock cleavage (fracture); and 

 gloss on fracture surfaces of the rock. 



' F. H. Lahee, "Relations of the Degree of Metamorpbism to Geological Structure 

 and to Acid Igneous Intrusion in the Narragansett Basin, R.I.," Am. Jour. ScL, (4), 

 XXXm, 249, 354, 447. 



2 Ihid., p. 355. 



