306 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



immediately in contact with trap, basalt, and analogous rocks, 

 do not always present the same appearance as at a distance 

 therefrom : and since the latter rocks, in all probability, are 

 of igneous origin, it has been concluded that this difference 

 has been caused by these rocks having been brought into 

 contact with the strata in a state of ignition ; whereby the 

 sedimentary deposits have been variously affected, according to 

 their composition. This view of the subject is further corro- 

 borated by the fact that these changes, with the exception of 

 dolomite, are such as might be produced by heat. 



There is, however, a most important consideration arising 

 out of this investigation, which must not be overlooked. 

 Admitting that these changes may have arisen by the presence 

 of intensely heated trap rocks, how comes it to pass that a like 

 cause has not always produced a corresponding effect ? How 

 is it, if these rocks have been intruded among the strata in a 

 state of ignition, that they have not equally altered the same 

 rock throughout their entire course ? For it is not easy to 

 conceive the reason why the basalt- dikes in Ireland should 

 change the chalk only in insulated patches ; or why dikes of 

 the same nature sometimes reduce coal into coke or cinder, 

 and at others traverse it without producing any alteration ; 

 or why the sandstones and shales are not always converted 

 into horns tone, jasper, Lydian stone, and flinty slate, when 

 intersected by igneous rocks. That this anomaly really does 

 exist, might be proved by numerous details ; but it is a fact 

 so often noticed, that it may suffice to refer to the statement 

 above quoted from Berger, that the phenomena of induration 

 of strata by the contact of basalt, though very frequent, are 

 not universal ; and to the works of Macculloch, which, among 

 many other examples, record that, in Skye, large dikes of 

 basalt, running perpendicularly through the sandstone at 

 Stratnaird, have not affected the texture of the rock ; whilst 

 at Duntulm, in the same island, the sandstone at the points 

 of contact have assumed a jaspideous aspect. 



The same anomaly also obtains in the case of the dolomite 



