460 MR. A. HARKEE ON THE ERUPTIVE ROCKS IN THE 



VII. The DoLERiTE-DrKES. 



The last rocks to be described, and tbat very briefly, occur as 

 dykes of no great width cutting through the larger igneous intru- 

 sions of the district. These dykes are seen in various localities to 

 intersect the granite, the diorite, and the hornblende-diabase ; and 

 although I have not recognized them as yet in the diabase, this is 

 owing, in all probability, to the difficulty of distinguishing in the 

 field between the latter rock and the dolerite. The larger intrusive 

 masses and the dykes and sheets of diabase we have seen reason to 

 assign to the Bala age ; the dolerite is most likely referable to a 

 much later period. 



In hand-specimens the dolerite is of medium or fine grain, with 

 well-pronounced ophitic structure, and of a dark colour, becoming 

 greenish in the most weathered parts. It appears to consist of 

 felspar, magnetite, and augite, and the microscope does not discover 

 any other constituents. 



The magnetite, either in granules or in octahedral crystals, is one 

 of the first-formed products of consolidation, though occasionally 

 it is seen to be penetrated by some of the smaller felspars. Of 

 ilmenite there is no indication in the slides examined. 



The felspar is constantly of two distinct generations. Of these 

 the earlier is clearly anterior to the augite in the order of consoli- 

 dation. It offers elongated sections with a fine lamellation on the 

 albite-type and frequently Carlsbad-twinning in the same crystals. 

 Judged by the extinction- angles the felspars are between oligoclase 

 and labradorite. The later generation of felspars occurs in more 

 equidimensional crystals, with rarely any trace of idiomorphic boun- 

 daries. They are never so closely lamellated as the earlier felspars, 

 but are marked by a strong zonary banding or shading in polarized 

 light. They belong to about the same stage of consolidation as the 

 augite. 



The augite, though pale brown, is never so nearly colourless as 

 the augite of the diabases and hornblende-diabases of the district. 

 It even shows in some cases a scarcely perceptible dichroism, chang- 

 ing from a rosy to a yellowish tint. The mineral, with the usual 

 cleavage well pronounced, occurs in crystalline grains or ophitic 

 plates moulding and enclosing the earlier felspars, and occasionally 

 moulding to some extent the later felspars also. 



The structure of the rocks is commonly ophitic, though there is 

 sometimes a tendency to pass into a partly granulitic type. 



Not recognizing geological age as an essential factor in lithological 

 classification, I have apphed the name dolerite to these rocks, because, 

 by the development of a later generation of felspars, they exhibit the 

 " porphyritic " structure of Eosenbusch, and are thus separable from 

 the diabases, in which the felspars belong all to one stage of consoli- 

 dation. Prom the type of diabase so largely represented in the Sarn 

 district these rocks differ, not only in this recurrence of felspar in a 

 second generation, but also in the characters of the felspars, in the 

 deeper colour of the augite, and its frequent occurrence in ophitic 



