PRE-CAMBRIAN COPPER-BEARING VOLCANICS 505 



granite porphyry, and finally to a porphyritic granite toward the interior. 

 The latter is the rock of Bald peak and of other principal hills of the 

 central portion of the Stoke Mountain area, as well as a part of the pre- 

 Cambrian of Weedon. The basic portion is less well preserved, and its 

 original characters can not be as precisely determined, as there are prob- 

 ably no original bisilicates now present. The decomposition products 

 and traces of the original structure indicate that the rock had, in some 

 cases at least, the characters of diabase, while in others it was probably 

 a porphyrite or andesite, rich in f erromagnesian constituents. Areas of 

 serpentine are occasionally found within the district occupied by this rock 

 which pass by sharp transition into hornblende porphyrite. While the 

 alteration to serpentine seems to be complete, no part of the orginal 

 rock being left, the serpentine has a somewhat different appearance from 

 that derived from the olivine-rich rocks of the larger serpentine areas of 

 the adjoining district. It is distinguished by the "grating" or "bar" 

 structure of serpentine derived from hornblende or augite instead of the 

 "mesh" forms resulting from the alteration of olivine. Small seams of 

 asbestos occur in this serpentine, but although several of the areas have 

 been prospected, no important deposit of that mineral seems to have been 

 found in them. 



The acid and the basic phases of these volcanics are, however, products 

 of separate irruptions, but are due to sharply denned magmatic differen- 

 tiation. This is well shown in several of the streams which drain the 

 southern part of Stoke mountain, notably on Eowe's brook. The rocks 

 may be considered as products of a single flow or of one flow for each 

 bolt. In the Sutton belt and, so far as is known, in the Lake Megantic 

 area also, there is evidence of no later volcanic action. In the Stoke 

 belt, however, later dikes occur somewhat frequently, but they are chiefly 

 of the camptonite and diabase classes. They cut the adjacent Trenton 

 sediments, and so belong to a series of rocks to be described later, rather 

 than to those of the present class, as they are not of pre-Cambrian age. 



The pre-Cambrian volcanics are apparently closely related to those of 

 South mountain, Pennsylvania, and other known localities to the south- 

 ward, and form a link of the more westerly of the two chains of early 

 volcanics that were described by the late G. II. Williams.* 



Possible pre-Cambkian Sediments 



Of the stratified rocks which are most closely associated with the vol- 

 canics referred to above the extremely chloritic portions are probably 



* Journal of Geology, January, February, 1894. See also "Ancient volcanics of South 

 mountain," by P. Bascom, Bull. U. S. Geol. Survey, no. 136. 



