BASALTIC ROCKS. 
125 
dike are like cinders or coke, while those close to 
it are converted into a substance resembling soot. 
In the island of Skt/, secondary sandstones are con- 
verted into solid quartz in several places where 
they come in contact with veins of trap. Rocks, 
however, are not always changed when in contact 
with volcanic dikes, owing perhaps to an original 
difference in their temperature, or the power of the 
invaded rocks to conduct heat, or to the quantity 
of water which they contain. Sometimes, also, the 
component materials are mixed in such propor- 
tions as prepare them readily to enter into chemi- 
cal union and form new minerals ; while in other 
cases the mass may be more homogeneous, or the 
proportions less adapted for such union. 
In this country the chemical effects of basalt and 
greenstone upon other rocks are witnessed in many 
places, though perhaps in a less striking manner 
than in some other parts of the world. The most 
obvious changes are observed when limestone is 
invaded by the trap. At Nahant, near Boston, we 
find argillaceous slate converted into flinty slate by 
the influence of greenstone, and in Charlestown, 
Massachusetts, the same rock* changes clay slate 
into hornstone. In the Connecticut Valley, at 
Rocky Hill, a sandstone is changed from a dark 
colour almost to a white ; and from a soft texture 
to the hardness nearly of flint, and is, moreover, 
iilled with vesicles,! giving it a highly porous char- 
acter. The same phenomenon can be seen on the 
east side of Mount Tom,^ in Northampton. Sand- 
stone also, in contact with the trap, is often found 
to assume a columnar form. The same fact was 
•observed by Dr. MacCuUoch in the hearthstone of 
a blast furnace. 
We have briefly alluded to the dislocation of strata 
by the injection of volcanic matter from beneath. 
* Professor Hitchcock. t Professor Webster, 
t Professor Silliman. 
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