PETEOGRAPHIOAL CHARACTERS OF MANSFIELD SLATE. 57 



is sometimes present. The ferro-magiiesian minerals develop chiefly from 

 the alteration of the feldspar, and from the finer detritus which is presmiied to 

 have existed between the grains. As a consequence, the secondary minerals 

 lie between the original grains. Many of the quartz grains are enlarged, 

 and here the secondary minerals are included in tlie new areas of the enlarged 

 grains. In numerous cases the new quartz occupies about as much space 

 as the original grains themselves. This shows very clearly the porous 

 character of the original sandstone. All original grains of the rocks show 

 signs of extensive mashing. Some specimens contain a large amount of 

 tourmaline in long slender crystals, which penetrate both the feldspar and 

 the quartz grains. The presence of tourmaline is especially interesting as 

 indicating that these sedimentaries may have been subjected to a certain 

 amount of fumarole action. According to the proportion in which the 

 various minerals have developed, we obtain sericite-, actinohte-, or chlorite- 

 schists produced from the graywackes. 



CLAY SLATE AND PHYLLITE. 



The clay slates are dull and lusterless and are black, olive green, or 

 red in color. They are usually impregnated with more or less iron pyrites 

 in large macroscopical crystals. One can distinguish in them quartz, white 

 mica, a few needles of actinolite, rutile, hematite, with a small proportion 

 of a dark ferruginous and carbonaceous interstitial material. 



The amount of iron which these clay slates contain varies considerably. 

 In some, hematite is present in such quantity ^s to cause the slates to be 

 appropriately called hematitic slates. Such, for instance, is the one forming 

 the foot wall of the Mansfield ore body. The iron oxide gives to the slates 

 a very bright red color where they are weathered. These weathered 

 hematitic slates are very commonly known in the district as red slates, or 

 as "paint rock" or "soapstone," though rocks of very diff"erent character are 

 also at times designated by these names. 



The phyllites have a silky luster and a bluish-black color. They are 

 composed essentially of white mica quartz, some feldspar, innumerable 

 minute crystals of rutile and dark feiTuginous specks. These seem to differ 

 from the rocks called here clay slates only in that they are more completely 

 crystalline, the interstitial material of the slates having disappeared. 



