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DYNAMICAL GEOLOGY. 



dikes of igneous rock. The fact indicates for the granite-like veins, 

 that they are the fillings of fissures in hot rocks, while a period of 

 metamorphism was in progress, and not in cold rocks like trap ; and, 

 for any others to which this last statement does not apply, that they 

 are fillings made by gradual deposition, and not by injection in a state 

 of fusion. 



4. Origin of the Fissures. — (1.) When the veins are confined to a 

 single bed (or are gash-veins), the fissures or cracks are usually due 

 to contraction, either from drying or from cooling. The purer or more 

 even-grained massive rocks, crack cross- wise, when the laminated beds 

 which intervene do not — the former contracting as a whole and in 

 any direction, but the latter mostly vertically. In the section of a 

 nearly black semi-crystalline, highly tilted limestone, near Husted, 

 Dutchess County, N. Y., the even-grained layers, two to four feet 

 thick, are transversely gashed, and the cracks which narrow toward 

 either surface, are filled in with calcite, making white veins, while the 

 less pure laminated layers between are solid. 



(2.) Fissures for the deeper veins have been made by subterranean 

 action or movements ; and their walls often bear evidence of the move- 

 ments in the smoothing, or polishing, and grooving of the surfaces 

 (making slickensides, in miners' language) from mutual abrasion. 

 These fissures, even the largest, are not often continuous openings for 

 miles, but rather series of fissures along a common direction. This 

 feature is illustrated in the cut on page 19. Often, as there repre- 

 sented, two or more parallel ranges exist with the successive parts 

 overlapping, and these parts may make an advancing or receding 

 series ; and transverse fissures may be of cotemporaneous formation. 

 (On the origin of fissures see beyond.) The above points are well 

 exhibited in the map (from Percival) on page 20, representing the 

 Mesozoic trap-dikes of the Connecticut valley, a careful study of 

 which is here recommended. Each line of trap on the map corre- 

 sponds to a fissure ; and the interruption and overlapping of the fissures 

 in a series are finely illustrated. 



(3.) Many of the faults in veins were made at the time when the 

 fissures were opened, shovings having accompanied the fracturing ; so 

 that even the faulting of intersecting veins is not alone evidence of dif- 

 ference in time of formation ; and those that were not made simul- 

 taneously may have been made with but a short interval between, 

 within the same epoch of disturbance. 



(4.) Veins are most common in metamorphic rocks, because a time 

 of metamorphism is one also of disturbance and upturning. But 

 periods of great fracturings have not always been times of metamor- 

 phism, and those of deepest fractures occurred in the Tertiary era. So 



