506 THE MARQUETTE IKON-BEARmG DISTRICT. 



be but local jiliases of the latter rock. There is no evidence of contact 

 action in them. The epidotization noticed around their edges is but one 

 exhibition of the general epidotization which much of the mass of the knob 

 has suffered. Nests and veins of epidote are common at the eastern end of 

 the westernmost knob, and, so far as known, they bear no definite relation 

 to the occun-ence of the "inclusions." 



THE DIKES. 



The typical dikes intrusive in the pre-Clarksburg rocks can not be 

 sharply distinguished from the boss-like dikes that have already been 

 described. They vary in width from a few inches to 60 feet, and so are 

 distinguished from the larger masses in size. Moreover, their walls are par- 

 allel and their courses usually straight, and these features again distinguish 

 them from the rocks that constitute the knobs. However, the small dikes 

 often are but apophyses of the boss-like dikes, and therefore they are but 

 portions of the latter, from which they do not differ in any essential respects. 

 The rocks that originally composed them were of the same nature as the 

 materials of the larger masses; at present they differ from the latter to a 

 slight degree in consequence of their greater proneness to alteration. 



While many of the small dikes are composed of greenstone identical 

 in composition with the materials of the knobs, the majority consist of the 

 highly schistose and much altered rocks which constitute the "diorite- 

 schists," "chlorite-schists," "soapstones," and "paint-rocks" of the miners. 

 The dikes of this kind are sometimes offshoots of the great knobs of green- 

 stone; at other times they appear as isolated bodies, which, however, in all 

 probability are connected underground with the bosses or with their down- 

 ward extensions. 



The dike rocks are in all respects so similar to the boss rocks that no 

 doubt could arise as to tlieir intrusive nature even were their field relations 

 not clearly those of intrusives. Every g-radation exists between the most 

 schistose dikes and those which still ])reserve, faintly it is true, the diabasic 

 structure. Tliese obscure diabases are identical with similar rocks forming 

 the knobs, and the latter may be clearly traced into true diabases, in which 

 augite may still be detected. None of the chlorite-schists and none of the 



