356 F. H. Lahee — Metamorphism and Geological Structure. 



The different phases may all occur in one locality, or even in 

 one seam, especially in regions of varying intensity of folding 

 (e. g., Aquidneck Island) (see tig. 21, p. 365) ; but where the de- 

 formation is extreme (western coast belt), only the higher stages 

 are found. The degree of metamorphism, then, is dependent, 

 at least in part, upon the amount and kind of deformation. 



Pelites. — According to their content of carbonaceous matter, 

 the pelites, representing original muds and clays, range in 

 color from black to light gray or greenish. They occur chiefly 

 in the Kingstown and Aquidneck formations. With the 

 darker phases coal seams are often associated. 



In their least metamorphosed condition (Stage A) these 

 rocks (shales) are dull and they break with rough, irregular 

 surfaces — usually more easily parallel to the bedding. The 

 finer the texture and the more abundant the plant remains, the 

 more perfect is this primary cleavage. The principal minerals, 

 as observed with the microscope, are carbonaceous powder ; 

 quartz, in minute, angular fragments, sometimes with undulose 

 extinction ; and a very little feldspar, sericite, and ilmenite. 

 All but the sericite are clastic in origin. There is no distinct 

 parallelism in the arrangement of the constituents, unless 

 it is in a streaking of the carbonaceous matter parallel to the 

 bedding. 



Low metamorphism (Stage B) is indicated megascopically by 

 a fair secondary cleavage, by a gloss on the cleavage surfaces, 

 due to a parallel arrangement of sericite laths, and by distor- 

 tion of fossils if these are present. Microscopic examination 

 shows the rock to have a composition similar to that of speci- 

 mens in Stage A ; but here the ilmenite is metarnorphic in 

 origin, as seen by its relations to the other constituents (see 

 fig. 1), the sericite is more abundant, and a little chlorite 

 occurs. The quartz grains have suffered more granulation and 

 slicing, particularly in the coarser rocks, and have sometimes 

 been squeezed out into lenticular aggregates (see figs. 4 to 10). 

 The parallel arrangement of these elongate quartz grains and 

 aggregates and of a large proportion of the sericite laths is the 

 cause of the secondary cleavage. The ilmenite has no definite 

 orientation. 



In the pelites of Stage C the sericitic gloss, just described, 

 has been enhanced in brilliancy by the richer development of 

 this mica; the secondary cleavage is usually more perfect; and 

 in most cases there has been anamorphic chemical rearrange- 

 ment with the consequent growth of 'knots' or metacrystals.* 



* A name used hy Lane in reference to phenocrysts in nietaniorphic rocks, 

 the metacrysts being of later origin than the groundmass. See Lane, A. C, 

 Studies of the Grain of Igneous Intrusives, Bull. Geol. Soc Am., xiv, 369, 

 1903. 



