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 v^ith 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 by 
what takes place in the vicinity of volcanoes. Before the con- 
fined vapour that afterward issues through the crater finds a 
vent there, the surface of the ground in the vicinity of the vol- 
cano is frequently upheaved, and fissures of great extent are 
made, into which melted lava is sometimes forced, which, on 
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 
