rARBON RIDGE AND SPUING HILL GROUP. 29 



Embraced within these lines of faulting only Carboniferous beds are exposed, 

 whereas the inclosing outer walls on botli sides consist of Silurian rocks 

 traceable the entire length of the mountains except where concealed by 

 volcanic overflows. Fissures along these fault lines have served as conduits 

 for extravasated lavas, through which have poured out, either upon one side 

 or the other, vast accumulations of volcanic material, for nearly the entire 

 length of the mountains. So extensive have been these flows over the 

 Carboniferous rocks that not oidy have the fault planes become obscured, 

 but large areas of the sedimentary beds lie concealed beneath the lavas, 

 while in the region of the Hoosac Mountain they have so spread out 

 over the country as to completely bury all the underlying rocks between 

 the two faults. Naturally such an amount of volcanic energy displayed 

 all along the line has broken and dislocated the strata, caused minor fault- 

 ings and displacements, and over much of the area rendered it difficult, if 

 not impossible, to work out the structural relations of the exposed beds. 

 Many fractures and breaks in the inclosed rocks, although not of any great 

 magnitude, are frequently sufficient to render any precise measurement of 

 the beds impossible, the amount of faulting being undeterminable. On 

 the other hand great blocks of strata have been tilted up at high angles 

 with only slight disturbances, affording fairly good cross-sections. 



The volcanic rocks separate the sedimentary beds, which otherwise 

 would form a continuous body, into two or more distinct areas, the northern 

 known as the Spring Hill group and the southern as the Carbon Ridge, 

 while between them lies a much smaller area of limestones every where sur- 

 rounded bv eruptive rocks. The middle area serves in a measure to connect 

 the other two, the same beds found here occurring both north and south. 

 Across the southern end of Spring Hill, where the strata are less dis- 

 turbed than elsewhere, the limestones present a synclinal fold whose axis 

 lies on tlfe west side of the ridge east of Spring Hill. Adjoining the 

 Hoosac fault lies a low, narrow ridge separated from the main body of lime- 

 stone by a north and south fault, beyond which the limestones on Spring 

 Hill dip easterly at an angle of 30, the beds on the opposite side of 

 the fold attaining angles as high as 60 westerly. Measured on the line 

 of the main section there are about 3,400 feet of limestones included 



