OVERLYING AND TRAP ROCKS. 127 



nuter branches into the surrounding strata, or else co- 

 alescing and being again separated ; neither of which 

 true strata ever do : while, lastly, they are found en- 

 tangling fragments of the surrounding rocks, which 

 remain insulated in them, under those fractures, in- 

 curvations, and changes of quality, which have been 

 so often noticed. It is almost unnecessary to say that 

 this description equally applies to the supposed alter- 

 nating beds of porphyry among the older rocks ; but, 

 among these, it appears to be a less common occurrence. 

 These parallel veins explain a not unfrequent case in 

 which trap presents a still more deceptive appearance 

 of stratification, by occupying the surface of the coun- 

 try and maintaining an apparently equal thickness, 

 with a general conformity to the secondary strata on 

 which it lies. Salisbury craig near Edinburgh is a 

 well known example of this; but a much more 

 perfect one occurs at the Garroch head in Bute. It 

 is plain that this has arisen from the loss of the su- 

 perior and secondary strata in which these beds had 

 once been included; and thus the accuracy of their 

 forms is explained, together w r ith their present supe- 

 riority of position. And any doubt of the truth of this 

 explanation is removed, by actually tracing the waste 

 of the strata in such cases, and by finding portions of 

 them still remaining after the greater part had disap- 

 peared. 



There is one distinct case, however, in which these 

 rocks may and do occur in the shape of strata, without 

 forming veins, or having intruded among the strati- 

 fied rocks. I have elsewhere shown that the effect of 

 the proximity of trap to shales is such as to convert 

 them into substances resembling basalt; and thus real 

 basalts are sometimes interstratified with siliceous 

 schists, with cherts, and with jaspers, derived from the 



