5 l6 J. F. KEMP AND J. G. ROSS 



two, one of which is three inches wide, the other many feet. 

 The latter makes as clean cut a diaphragm with respect to the 

 heading as would a stone flag set on edge. At a point about 

 one-third its length to the southeast, there are again two parts 

 above ground. Apparently as the dike rose from the depths 

 toward the surface, it sent off stringers, or itself forked into two 

 or more parts. At the Pittsburg seam it expanded to its 

 greatest known width. At the same time it coked the coal, 

 and its effects may be detected for fifty feet on each side in the 

 seam. At the edges of the dike the coked coal has often become 

 involved in the igneous rock, which, itself, is finer grained from 

 the chill. 



On the outcrop, the rock is weathered and nearly brown. It 

 breaks in spheroidal masses which are very tough and hard to 

 fracture. Where freshest in the mines it is blackish gray and 

 decidedly porphyritic. The phenocrysts may reach two or three 

 centimeters in diameter and are chiefly a pale green mineral 

 breaking along an even but not perfectly smooth surface. Small 

 phenocrysts of the reddish brown biotite characteristic of the 

 basic rocks may be detected with the eye, and quite large but 

 rounded masses of magnetite are scantily set in the matrix. 

 In its finer texture the rock has a granular aspect due to the 

 predominant but rounded grains of some mineral not recog- 

 nizable with the unaided eye. 



Under the microscope the rock is found to be much altered in 

 all specimens, but in the freshest the original minerals can be de- 

 termined or inferred with much certainty. As in the other 

 similar cases biotite has resisted weathering much the best of 

 all, and its brownish yellow crystals seem in many cases slightly 

 if at all affected. It varies in size from small plates o.i mm. in 

 diameter up to crystals two or three millimeters across. It is 

 strongly pleochroic, golden- brown to colorless, and has a 

 visible but small angle between the optic axes. 



Olivine is the most abundant component, but it has become 

 altered almost beyond recognition in nearly all cases. Its out- 

 ward shape, however, is characteristic, and in several instances 

 fresh nuclei have been detected which gave a positive result, when 

 optically tested. The olivine is itself so nearly colorless, and its 



