VULCANISM OP THE PERIOD. 165 



tured strata, and the filling-up of the rents and fissures 

 with igneous rock-matter. "We say " interstratified green- 

 stones," for some observers speak of "intrusive green- 

 stones," as if, during the vulcanic paroxysms, such igneous 

 rocks had been forced for miles in every direction between 

 the separated strata ! There are, no doubt, intrusive masses 

 among the strata of every formation, but these are general- 

 ly limited in extent, irregular in form, and bake or harden 

 alike the immediately underlying and overlying beds. The 

 interstratified greenstones, on the other hand, affect only 

 the strata on which they rest, and to regard such wide- 

 spread lava-like sheets as intrusive, is simply an absurdity. 

 Indeed, almost every feature of the vulcanism of the period 

 has been perfectly preserved to us, and the imagination has 

 little difficulty in recalling the broad bays and estuaries of 

 the Carboniferous ocean, studded with their cones and 

 craters of eruption here ejecting showers of dust and 

 ashes, there discharging floods of molten rock-matter, and 

 ever and again the whole shaken and fractured by earth- 

 quake energy and convulsion. Part of this vulcanism was 

 contemporaneous with the deposition of the sediments, as 

 proved by the interstratified greenstones and trap -tuffs, 

 but part also happened long subsequent to the solidifica- 

 tion of the strata, as seen in the faults and dykes of 

 injected matter that intersect the whole thickness of the 

 system ; but whether contemporaneous or long subsequent, 

 it forms one of the most remarkable features in the coal- 

 formation, engaging the closest study of the geologist, and 

 exercising all the ingenuity of the miner and engineer. 

 If the reader could picture to himself the district in which 

 he resides fractured by earthquake convulsion here a 

 portion thrown many fathoms up, there another portion 

 thrown many fathoms down, and the rents between filled 

 with solidified lava he would have before him precisely 



