48? 



INTRUSION OF TRAP BETWEEN" STRATA. [Oh, XXIX 



a variety of cases, shale being turned into flinty slate or jasper, limestone 

 into crystalline marble, sandstone into quartz, coal into coke, and the 

 fossil remains of all such strata wholly and in part obliterated, it is by no 

 means uncommon to meet with the same rocks, even in the same dis- 

 tricts, absolutely unchanged in the proximity of volcanic dikes. 



This great inequality in the effects of the igneous rocks may often arise 

 from an original difference in their temperature, and in that of the entan- 

 gled gases, such as is ascertained to prevail in different lavas, or in the 

 same lava near its source and at a distance from it. The power also of 

 the invaded rocks to conduct heat may vary, according to their composi- 

 tion, structure, and the fractures which they may have experienced, and 

 perhaps, also, according to the quantity of water (so capable of being 

 heated) which they contain. It must happen in some cases that the com- 

 ponent materials are mixed in such proportions as prepare them readily to 

 enter into chemical union, and form new minerals ; while in other cases 

 the mass may be more homogeneous, or the proportions less adapted for 

 such union. 



We must also take into consideration, that one fissure may be simply 

 filled with lava, which may begin to cool from the first ; whereas in 

 other cases the fissure may give passage to a current of melted matter, 

 which may ascend for days or months, feeding streams which are over- 

 flowing the country above, or are ejected in the shape of scoria; from 

 some crater. If the walls of a rent, moreover, are heated by hot vapor 

 before the lava rises, as we know may happen on the flanks of a volcano, 

 the additional caloric supplied by the dike and its gases will act more 

 powerfully. 



Intrusion of trap between strata. — In proof of the mechanical force 

 which the fluid trap has sometimes exerted on the rocks into which it 

 has intruded itself, I may refer to the Whin-Sill, where a mass of basalt, 

 from 60 to 80 feet in height, presented by a, fig. 632, is in part 



Fig. 682. 



Trap interposed between displaced beds of limestone and shale, at White 

 Force, High Teesdale, Durham. (Sedgwick.*) 



wedged in between the rocks of limestone, b, and shale, c, which have 

 been separated from the great mass of limestone and shale, d, with which 

 they were united. 



* Camb. Trans. voL ii. p. 180. 



