F. H. Lahee — Metamorphism and Geological Structure. 355 



new and entirely different minerals in other cases. Every 

 stage in the process, almost from the beginnings of the change 

 to the highest metamorphism, is exhibited by these sediments 

 in different parts of the Basin. 



Study of the Specimens. — In our treatment of the petrol- 

 ogy, we shall consider the metamorphism as having four 

 degrees, called Stages A to D. The metamorphism is incip- 

 ient in Stage A ; distinct, but rather low, in Stage B ; consid- 

 erable to high in Stage C ; and at a maximum in Stage D. 

 We shall describe in order the coals, the pelites (shales, slates, 

 and mud schists), the psam mites (sandstones, grits, arkoses, and 

 their metamorphosed derivatives), and the psephites (conglom- 

 erates and conglomerate gneisses). 



Coals. — While the coals form a very insignificant proportion 

 of the whole stratigraphic series, they are important in that 

 they illustrate certain principles which apply, though less 

 clearly, to the other sediments. The least altered specimens 

 (Stage A) are soft and friable and crumble into small regular 

 polyhedra. There is a direction of easy splitting parallel to 

 the bedding (primary cleavage). So slight has been the deforma- 

 tion that most delicately ribbed impressions of Cordaites have 

 been hardly distorted. 



With an increase in the metamorphism (Stages B and C), 

 the rock becomes less brittle, less friable, and somewhat harder 

 and denser. A secondary cleavage is developed, with flat or 

 undulating surfaces, and these surfaces are often slickensided 

 or have a bright gloss which is due to internal rock movement 

 under pressure. 



The extreme stage of metamorphism (Stage D) is found near 

 large, massive quartz veins which, as will be explained subse- 

 quently, are abundant in certain localities. Here the secondary 

 cleavage is extremely thin and contorted and there is a great 

 deal of slickensiding and glazing. Chemical analyses show 

 that, with this increase in metamorphism, the ratio of fixed 

 carbon to volatile constituents becomes greater. The coals 

 pass from anthracite to graphite or 'plumbago'. None of the 

 phases are marked by the occurrence of minerals typically of 

 metam orphic origin. 



In their present condition the coal seams are far from being 

 uniform in thickness, for the compression to which they were 

 subjected during the folding had the effect of thinning where 

 the strain was at a maximum and of thickening where it was 

 at a minimum, so that they are now ribbed or 'rolled.' This 

 distortion involved shearing and brecciation. Shearing and its 

 consequent high degrees of metamorphism are characteristic of 

 the thinner portions ; and brecciation is more common in the 

 thickened portions in which lower stages of metamorphism 

 prevail. 



