HOOSAC MINE. 113 



dikes. Tluro occur here some curious bands of a dark brecciated (juartzite 

 made u\) oi' chert autl jasper, in fragments firmly cemented together and 

 brilliantly colored by secondary alteration. The cementation pro])ably fol- 

 lowed the inhltration of silica, -which took place during the volcanic ])eriod. 

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

 in narrow dikes, the surface exj)osures of which are nuicli decom])osed and 

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

 dikes of perfectly fresh rock were observed. ^linei-s searching for ore 

 bodies along the outcrops of these decomposed rocks ha\e explored them in 

 a Avav to permit of their general course and mode of occm-euce being made 

 out. From underground exploration thei'e 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 tlie 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 uj)lift, following the direction of the main 

 Hoosac fault. Although nnich decomposed, the andesitic character of 

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

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

 clinic. The i-hyolite 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, liaAing 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 

 rmis westward into the mountain. All the mine workings lie in (juartzite, 

 the ore bodies encountered being found in connection Avith the intriisive 

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

 time precious metals to tlie value of S500,000. Continued exploration 

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



Northward of Hoosac Mountain the Pogonip limestone maintains, as 



MON XX 8 



