128 



DIKES AND FAULTS. 



" In reference to this inquiry, it is most important 

 to bear in mind the original condition of the strata 

 submerged under the ocean or in deep lakes. We 

 may form some idea of what this condition was, by 

 what we may sometimes observe at the present day 

 in beds of calcareous tufa at the bottom of lakes or 

 rivers recently laid dry. Such beds often yield to 

 the pressure of the finger ; but, when exposed to the 

 atmosphere, they harden and form building-stone. 

 Even the strata of sandstone in deep quarries may 

 often be crumbled within the hand ;* yet, after long 

 exposure to the air, the same stone yields with dif- 

 ficulty to the chisel of *the mason ; indeed, the soft- 

 ening power of water is sometimes manifest even 

 in rocks believed to be of igneous origin." 



The above view of the subject is undoubtedly a 

 correct one, and will explain all the difficulties hith- 

 erto proposed in relation to it. If the basaltic dikes 

 were even protruded after the containing rocks 

 were consolidated, but before they were elevated 

 above the ocean bed, we think it more than proba- 

 ble that such beds, saturated with water, would pre- 

 sent but little resistance to the agitation of the 

 ocean, when they were suddenly raised above the 

 lower submarine ground. 



We may, therefore, safely conclude, that the dis- 

 appearance of the strata upraised by faults is owing 

 to the soft and yielding condition! of the submerged 



* Much of the marble of Berkshire county, Massachusetts, is 

 flexible when first taken from the quarry, but loses this proper 

 ty by exposure. 



t The formation of basaltic dikes is sufficiently explained bj 

 what takes place in the vicinity of volcanoes. Before the con 

 fined vapour that afterward issues through the crater finds i 

 vent there, the surface of the ground in the vicinity of the vol 

 cano is frequently upheaved, and fissures of great extent an 

 made, into which melted lava is sometimes forced, which, 01 

 cooling, forms a wall or dike in every respect similar to a basal 

 tic dike. During an eruption of Vesuvius in 1794, a vent of this 

 kind was formed near the bottom of the mountain, 2375 feet in 

 length and 237 in breadth, which became filled with compact 



