BASALTIC ROCKS. 



125 



dike are like cinders or coke, while those close to 

 it are converted intc a substance resembling soot. 

 In the island of Ski/, 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, 

 filled with vesicles,f giving it a highly porous char- 

 acter. The same phenomenon can be seen on the 

 east side of Mount Tom,J 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. MacCulloch 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. 



$ Professor Silliman. 



