248 GEOLOGY OF THE EUREKA DISTRICT. 



impossible that such knobs should have been formed and later have been 

 removed by erosion without leaving some evidence of overflow along the 

 line of the dikes. 



This system of dikes upon Prospect Ridge presents certain geological 

 characteristics of interest bearing upon the mode of occurrence of erupted 

 material. Throughout they show a great similarity in mineral composi- 

 tion and petrographical habit, and when fresh in every way resemble the 

 unaltered rocks along the Ruby Hill fault. These latter lavas have been 

 shown elsewhere to have been erupted at the same time and under similar 

 conditions with the rhyolite of the Hoosac fault. Indeed, the Ruby Hill 

 fault is simply a prolongation of the main fault. Evidences of alteration 

 and metamorphism of the limestones and shales through which the erupted 

 material passed are by no means easy to detect, the encasing walls showing 

 scarcely any evidence of the effects of heat derived from ascending lava 

 currents. These dike rocks being narrow bodies have cooled rapidly and 

 imparted little heat to the limestones. Mining exploitations have frequently 

 encountered these intrusive bodies hundreds of feet below the surface, but 

 neither at the top nor underground do they exhibit structural features in 

 any way different from the larger bodies. The only marked feature in 

 which these dike rocks differ from the extrusive lavas of Pinto and Gray 

 Fox peaks is shown by the absence of flow structure due entirely to their 

 manner of occurrence, and in no way dependent upon either their chem- 

 ical or mineralogical composition. As regards the degree of ciystalliza- 

 tion, they exhibit characters identical throughout and similar to the material 

 erupted at the surface along the principal lines of faulting. 



On all the great lines of orographic fracture along which both acid 

 and basic lavas have emanated, the amount of volcanic material reaching 

 the surface has varied greatly at different points. In certain localities 

 they have piled up to such an extent as to form prominent hills and 

 landmarks, but their mode of occurrence is precisely the same as those 

 where the lavas have only accumulated in narrow belts along the fissures. 

 Such masses as Pinto Peak, Purple Hill, and Gray Fox Peak are similar 

 piles of lava, uniform in character, only varying in size according to the 

 amount thrown out at each locality. In the same way Richmond Moun- 



