Essay introductory to Geology. 15 



Sometimes detached masses of a given series of strata will constitute 

 insulated hills, now separated from the main range of the said strata, which 

 must, however, originally have spread over the interval whence they are 

 now removed. These phenomena seem to attest the very destructive 

 forces which must have acted on the face of our planet. 



§ 4. The phenomena of active volcanoes are well known, and need not 

 here be repeated. But, besides the volcanos still active ; central France, 

 the banks of the Rhine above Bonn, and many other places, present 

 regular craters, torrents of lava, and whole districts covered with volcanic 

 ashes j so as to leave no doubt that the same igneous action has here also 

 originally prevailed, though the fires be now extinct. But besides these 

 undoubted volcanos, we also find irregularly breaking into and traversing 

 the regular strata, and filling up vertical fissures uith masses, which are 

 called dykes. If we examine the substance of these unstratified and 

 interfering rocks, (which are called trap-rocks,) we shall find it closely 

 agreeing in mineral composition with the undoubted volcanic rocks. If we 

 look at the effect which such trap-dykes produce on the strata which they 

 intersect, we shall find such as would have resulted from the contact of 

 masses of heated lava indurating the soft strata, charring the coal beds, 

 &c.* Be it observed, that we do not now theoretically assert that fire has 

 been the agent producing these effects ; we simply describe the phenomena 

 when we say that they are such as would have resulted from igneous 

 agency. Among the primitive rocks, granite performs a part quite analo- 

 gous to that which trap performs among the secondary j and some varieties 

 of granitic rocks insensibly graduate into some varieties of trap. All the 

 phenomena indicate a close relation of analogy between these formations. 



• When trap-dykes indeed, intersect limestone, they do not turn it to lime ; but 

 on the contrary, convert it into crystalline saccharine marble, as may be well seen 

 where the trap-dykes cut the chalk, in the north of Ireland, liut it has been found 

 by experiment, by Sir James Hall, that this is exactly the effect produced by heating 

 liinestone, under the pressure of a column of water. I have seen the specimens he 

 produced by thus heating lime, at the bottom of a tube of gun barrels, filled with 

 water, and placed vertically, and they exactly resemble the altered Irish chalk, at its 

 contact with the trap-dykes. The effect here produced, therefore, is such as would 

 have resulted from the intrusion of igneous masses, ejected beneath the pressure of 

 an incumbent ocean, by a sub-marine eruption. 



