448 ME. A. HAEKEE ON THE EBUPTIVE EOCKS IN THE 



specimens and the slides. The process of alteration has proceeded 

 further in the rocks on the edge of the district than in those of the 

 central part. 



The rock exposed near the bridge where the main-road crosses 

 the stream may be taken as a type of the massive diorite. The 

 microscope shows but little of the black iron-ores, but there is a 

 considerable quantity of sphene in brownish granules. These 

 granules, with cleavage-traces and irregular fissures, are aggregated 

 in small patches and strings between the grains of felspar and 

 hornblende. The felspar is partly in granular patches, too much 

 decomposed to show any structure. The hornblende is of a greenish- 

 brown or brownish-gTeen colour, and gives for the three axes of 

 elasticity : — a, very pale to almost colourless ; /3, rather deep brown 

 with greenish tinge ; y, slightly deeper tint of greenish brown. 

 There are also very pale greenish actinolite-looking shreds amongst 

 the felspar-grains. 



Xear Plas Llangwnadl the rock is more altered, and has in places 

 a quasi-porphyritic appearance, owing to the granular felspar forming 

 large patches. Here there is very little of the opaque iron-ore, but 

 sphene is more abundant. The felspar has undergone a seemingly 

 saussuritic change, but a little of it still preserves the original 

 structure and appears to be labradorite. 



Still further down the stream, approaching the boundary of the 

 area, the schistose appearance becomes more pronounced. A slide 

 from this portion shows no new characters, except that the black 

 iron-ore has entirely disappeared. 



The mode of occurrence of all these rocks, both at Craig-y-fael 

 and near Llangwnadl, proves that they were intruded through the 

 granite, doubtless in the form of gabbro. The hornblende may be 

 supposed to be entirely of pseudomorphic origin, the change being 

 probably assisted by the mechanical stress to which must be ascribed 

 the schistose structure of the rocks about Llangwnadl. At Craig- 

 y-fael, where the gabbro was protected by a large mass of surround- 

 ing granite, the amphibolization is still imperfect. In the other 

 mass, where the conversion is complete, the schistose character 

 becomes more marked towards the boundary, and it may reasonably 

 be ascribed to the movements which produced the assumed fault. 

 In confirmation of this, it may be remarked that the strike of the 

 gneissic and schistose structures is the same as that of the fault, and 

 the schistosity is shared on the other side of the boundary by the 

 ashy-looking beds which represent, near Llangwnadl church, the so- 

 called green schists. The production of sphene in connexion with 

 this kind of metamorphism is too well known to require comment. 



The date of the gabbro and diorites is, then, more recent than that 

 of the granite, but earher probably than the fault ; the rocks may 

 be referred with some doubt to the Bala age. 



lY. The Diabase. 

 The main mass of diabase in the district forms the curious conical 

 hill named Clipiau-cilfinhir, half a mile west of Rhiw, rises in the 



