part 2] GEOLOGY OF THE TBEFBXW PTBITES deposit. 109- 



immediately above the ore-body. The other locality is the waste- 

 material thrown out of the old level on the 800-foot contour, 

 150 yards east of Blaen-y-wern. 



The dip of the shale is southward on the whole, and is- 

 approximately parallel to the inclination of the ore-body. As 

 one proceeds along the nearly horizontal pathway from the main' 

 (No. 2) adit to the top of the incline, a little below the 600-foot 

 contour, the dip of the shale increases to about 60°. Here, at the 

 head of the incline, we are about 240 feet vertically above the top> 

 of the igneous mass. In the slopes above the incline bands of ash 

 are found to occur in the shales at intervals : it is probable, there- 

 fore, that these ashy shales are older than the graptolitic beds, and 

 belong to the lowest division of the Lower Cadnant shales : that 

 is, the ' flags and shales with siliceous ashy bands ' of Miss Elles's- 

 classification. If so, there is a thrust bringing: them over the shales- 

 with graptolites (arcius beds). 



II. The Igneous Rocks. 

 (A) The Intrusive Rock below the Ore-Body. 



The upper boundary of the intrusive mass usually is apparently 

 conformable to the shales above, except in a few places where 

 the ore is missing, or where narrow veins of igneous rock jut into 

 the overlying shales. There is also in such places an appearance 

 of contact-alteration of the shales : for some 6 inches to 1 foot 

 of these, next to the igneous mass, have been hardened and have 

 lost their tissile structure, and they also contain crystals and nests, 

 of pyrites. Under the microscope no true metaniorphic change 

 can be seen, and the contact-alteration is, therefore, confined to the- 

 first stages of metamorphism. 



The intrusive rock is a diabase containing ophitic augite, 

 serpentine, chlorite, and ilmenite. Xear the ore one often sees. 

 pyrites in the rock. 



Additional evidence that the rock is intrusive is °*iven bv its. 

 thickness which, as already mentioned, is about 340 feet vertical 

 at the incline. 



In the old adit near the road the base of the intrusion is exposed, 

 and black shale seamed with quartz is seen beneath it. The adit 

 contains bad air: but Mr. Morris has penetrated some distance 

 into it, and has brought out a quantity of pyrrhotite from the 

 base of the igneous rock. A specimen is markedly magnetic. 



(B) The Volcanic Ash. 



Below the main (No. 2) adit there is a great fan of debris, 

 covering a large part of the hillside. Through the rubbish 

 emerges an occasional crag of ash. Fortunately, at the entrance 

 to Adit No. 3 (almost beneath No. 2) the ash is visibly thrust 

 over the black shale, while about 10 yards within the adit the ore 

 is reached. The thrust has a steep hade, and the effect is that the- 



