504 H. G. Smith — Basic Intrusions 



intrusion is certainly post-Llandovery. But there remains the 

 interesting question as to why the diabase has failed to cut through, 

 the limestone. We are compelled to suppose that a magma exercises 

 a careful selection ^ with regard to the rock invaded. In the case 

 here considei'ed the diabase magma readily penetrated the shales, 

 but the overlying limestone presented an almost impassable barrier. 

 Igneous material was injected along the bedding planes, and pockets 

 containing the diabase were formed in consequence of a boring action 

 on the part of the magma, while the areas occupied by a network of 

 felspar with interstitial calcite resulted from the intrusion of 

 a felspathic portion of the magma into a locally fused limestone area 

 and subsequent crystallization of felspar followed by that of calcite. 



All the evidence supports the theory that the diabase, when 

 intruded into its present position, was nearing the limit of its powers 

 of penetration. The still existing serrations of the upper edges of 

 the sills, the failure to cut through the limestone, the very feeble 

 metamorphism, and the fact that the silicates do not incorporate any 

 of the calcium from the adjacent or containing limestone, all point 

 in the same direction. 



The conclusion here arrived at as to the age of the diabase is in 

 direct opposition to that put forward by Mr. Woods' as a result of 

 his examination of the area to the south. He relies on the facts that 

 " nowhere do they (the diabases) pierce the Silurian beds ", and that 

 at " the section exposed in the quarry next Pen-cerig Lake, where 

 the diabase is seen in contact with both Llandeilo shales and the 

 Llandovery beds, the former are metamorphosed, the latter quite 

 unaltered". But we have seen reason to suppose that failure to cut 

 through a sediment is no proof that intrusion took place before that 

 rock was laid down, and it follows from the facts put forward that 

 striking metamorphic effects are not to be expected. 



If the post-Llandovery date of the diabase is accepted, then these 

 intrusions are brought into line with those of the Shelve area, where 

 Prof essor Watts ' has shown that the dolerites "come into contact 

 with and somewhat alter the Pentamerus limestones". 



Professor Fearnsides* concludes that the andesitic dolerites of 

 Arenig "are of the same general age". 



Dr. Harker,^ under the impression that the cleavage and plication 

 of the strata of Eastern Carnarvonshire were developed in pre-Silurian 

 times, assigned a Bala age to the diabases of that area, but Professor 

 Pearnsides^ points out that "with increase of knowledge the 

 supposed gap in the continuity of sedimentation has been filled up, 

 and now a Post-Silurian date for the cleavage is generally accepted". 



It appears, then, that this post-Llandovery intrusion of diabase or 



^ This power of selection is implied by Professor Watts in. his description of 

 the intrusions of the Shelve area (Proc. Geol. Assoc, vol. xiii. p. 342, 1894). 

 '■^ Loc. cit., p. 577. 

 ^ Loc. cit., pp. 339-40. 

 •* Q.J.G.S., vol. Ixi, p. 631, 1905. 

 ^ Bala Volcanic Series, 1889, p. 76. 

 ^ Geology in the Field, 1910, p. 803. 



