440 Dr. Mac Culloch on the Geology of 



nothing to add in explanation of them ; they merely suffice to 

 prove, that these substances like the schistose rocks, have been in a 

 state which admitted of bending ; the capricious and intricate con- 

 tortions visible in some of the figures (which were drawn with great 

 care) not admitting of any fissure of this form to be subsequently 

 filled either by injection or infiltration. One of the figures is per- 

 haps particularly worthy of notice, on account of the distinctness 

 with which it demonstrates the action of partial forces, by the re- 

 markable difference of curvature occurring in two veins so nearly 

 approximated. 



Burnt Island, 



A specimen of a compound vein is before the Society, which 

 traverses the well known amygdaloid of this shore. This vein con- 

 sists of a highly crystalline brown limestone, mixed with a ramified 

 compact earthy black basalt. These substances are so intermingled, 

 that it is scarcely possible to conceive that the basalt is posterior to 

 the limestone, or that a basalt vein should have found its way into 

 a vein of limestone without also traversing the amygdaloid. It is 

 probable that the whole vein is of simultaneous formation, and that 

 the substances have separated from each other in consequence of 

 those obscure chemical affinities which regulate the crystallizations 

 of compound rocks. This however is a subject deserving of fur- 

 ther investigation, as, if it is admitted, it will prove that the same 

 cause, whatever that be, may preside over the formation of basaltic 

 veins, and those veined limestone deposits in trap rocks, which are 

 called calc sinter, and are held to arise from the posterior effects of 

 a watery infiltration. 



In the same rocks a singular circumstance accompanies the im- 

 bedded calcareous nodules. Their surfaces are occasionally marked 



