part 2 GEOLOGY OF THE TREF1UW PTEITES DEPOSIT. 107 



No. 2, a scree 450 feet high forms a marked feature in the 

 landscape, and obscures a good deal of the geology. 



The ore resembles a bed, usually about 6 feet thick, but varying 

 somewhat, being thicker on the crest of folds and thinner on the 

 limbs. It separates an intrusive igneous mass below from black 

 shales above, which some poorly-preserved graptolites prove to 

 belong to the arctus subdivision of the Dicranograptus Shales. 

 The intrusion is a thick mass covering the whole hill-slope from 

 half way up the inclined tramway, at 3(50 feet above sea-level to 

 the alluvial flat below, giving a vertical depth of about 310 feet. 

 Fortunately, an old adit near the bottom of the incline proves 

 that the full thickness of the igneous mass is exposed, for black 

 shale is seen in the adit below the intrusive. 



Proceeding northwards, we see that the igneous rock is covered 

 by an overthrust mass of volcanic ash, the thrust having a curved 

 outcrop which is convex southwards and reaching the alluvial 

 flat about 70 yards south of the 4th milestone. The intrusion 

 passes out of sight beneath the ash, but is seen in Adit Xo. 2, 

 higher up the hill. On the north, the hill-slope is all volcanic 

 ash, until a fault, running approximately east and west, situated 

 about 200 yards north of the 4th milestone, cuts it off and brings 

 In rhyolite. 



The top of the hill is a plateau made up of intrusive dolerite, 

 which covers all the other rocks in turn. The same dolerite is 

 •also found in crags in the rhyolite area, near the road a little south 

 of the road-metal quarry. 



The map shows the outcrops of the various rocks and the section 

 illustrates their relations to one another. The various rocks may 

 how be described in a little more detail. 



I. The Clastic Kocks. 



The only sedimentary rocks that occur are hard, flaggy, black 

 shales, slightly micaceous and in part somewhat cleaved. At two 

 points numerous poorly-preserved graptolites were found, some of 

 which Mr. John Pringle has been able to determine as Diplo- 

 </ rapt us (Amplexograptus) arctus Elles & Wood, cf. Diplo- 

 graptus (Gh/ptograptus) teretiusculus (Hisinger), and Clima- 

 cograptus sp. Mr. Pringle adds : 



'The forms identified as Diplograptus (Amplexograptus) arctus E. & W. 

 ■occur in some profusion, and they suggest strongly that the shales occupy a 

 corresponding position to that of the Mydrim Limestone of South Wales.' 



In Miss Elles's 1 classification these beds belong to the zone of 

 Nemagraptus gracilis. 



In both localities the fossils were obtained from shale lying only 

 ■a short distance above the ore. In shale from Adit No. -i. 

 369 feet above O.D., the graptolites are seen on one side and 

 pyrites on the other, whence it follows that the specimen is from 



1 Q. J. G. S. vol. lxv (1909) p. 172. 



