128 OVERLYING AND TRAP ROOKS. 



action of heat on the original strata. And if I have 

 also proved the partial fusion and conversion of schists 

 into basalt, greenstone, or porphyry, such cases need 

 not be limited in extent; while it is easy to see that a 

 body of various strata, thus acted on, must exhibit re- 

 sults proportioned to their respective capacities for fu- 

 sion. The most fusible material becomes converted 

 into basalt or greenstone, or, under less heat, into 

 siliceous schist, the argillaceous limestones into chert, 

 and the sandstones into other analogous compounds; 

 while the infusible clays become jasper, or, like the 

 red iron clay so common among these rocks, retain 

 their loose texture. This hypothetical case actually 

 occurs in Sky, on an enormous scale ; and it is only 

 to extend the powers of this cause, even far within 

 the limits which it has obviously exerted in nature, to 

 explain many of the examples of stratification in trap, 

 to which the former solution does not apply. As a 

 striking example in point, I may again refer to the 

 porphyry near Campbelltown ; where the intermix- 

 ture of fragments of schist, and their gradual conver- 

 sion into the unstratified rock, are perfectly obvious. 

 A phenomenon precisely similar has been since ob- 

 served at Schernnitz in Hungary, and will, no doubt, 

 be oftener seen, as observers learn to follow where 

 they had never known how to lead. 



The last form in which the rocks of this family 

 occur, is that of veins. These abound equally in the 

 most ancient as the most .recent traps, and are often 

 indeed the only remaining traces of the former ex- 

 istence of extensive masses. Varying exceedingly in 

 size, the largest ones bear such a relation to the over- 

 lying masses, that I should have already noticed them 

 had I not considered that they would be better un- 

 derstood in this place. 



