HOOSAC MINE. 113 



dikes. There occur here some curious bands of a dark brecciated quartzite 

 made up of chert and jasper, in fragments firmly cemented together and 

 brilliantly colored by secondary alteration. The cementation probably fol- 

 lowed the infiltration of silica, which took place during the volcanic period. 

 Both hornblende-andesite and rhyolite penetrate the mountain, but mainly 

 in narrow dikes, the surface exposures of which are much decomposed and 

 in most instances so altered as to render a study of them impossible ; no 

 dikes of perfectly fresh rock were observed. Miners searching for ore 

 bodies along the outcrops of these decomposed rocks have explored them in 

 a way to permit of their general course and mode of occurence being made 

 out. From underground exploration there is reason to believe that but a 

 small part of the andesite dikes reach the surface, and these only in stringers 

 and offshoots from some parent body. Mapping the hornblende-andesite 

 exposures along the mountain, they are seen to follow a common course 

 approximately north and south, coincident with the lines of faulting and 

 the trend of the mountain uplift, following the direction of the main 

 Hoosac fault. Although much decomposed, the andesitic character of 

 these rocks can be readily made out from a study of their hornblendes and 

 glassy feldspars; the latter under the microscope are found to be all tri- 

 clinic. The rhyolite exposure just east of the Hoosac mine appears to be 

 a remnant left by erosion from the main body of the Hoosac fault outburst. 



The Hoosac mine, situated on the east slope of the mountain, is one 

 of the oldest mining properties in the district, having been located in 1869 

 and opened early the succeeding year. As it is the only mine in the dis- 

 trict found in the Eureka quartzite, it has much geological interest, and its 

 development has served at least to furnish data bearing upon the structure 

 of a singular mountain. A vertical shaft 200 feet in depth has been sunk 

 through the quartzite, from the bottom of which a level 300 feet in length 

 runs westward into the mountain. All the mine workings lie in quartzite, 

 the ore bodies encountered being found in connection with the intrusive 

 rocks. It is reported that the owners of the property took out in a short 

 time precious metals to the value of $500,000. Continued exploration 

 failed to maintain the high hopes first entertained of the mine. 



Northward of Hoosac Mountain the Pogouip limestone maintains, as 

 MONXX 8 



