21 DE. STAMP AND ME. WOOLDEIDGE ON THE [vol. lxxix, 



be intruded along the fault itself. There seems to be only one 

 small piece of evidence that bears on this question : the faulting 

 has been accompanied by a considerable amount of shattering and 

 subsequent infilling of the cracks by quartz. In fact, some of the 

 ' horn stones ' near the fault are riddled with small quartz- veins. 

 Now, the intrusion is also affected by this quartz-veining to some 

 extent : consequently, if it was intruded along the fault, it was 

 afterwards subjected to shattering and quartz-veining due to 

 renewed movement along the fault-line. In the field the intrusion 

 has a curious patchy appearance on a small scale, and also exhibits 

 a rough flow-structure. This appearance led me to name the rock 

 ' mixture-rock ' in the field ; but its hybrid nature has not been 

 borne out by detailed examination. The intrusion is seen in some 

 crags about 100 yards south-east of Pen-y-banc Farm; it is the 

 ' greenstone ' of Murchison. 



As our main object in this investigation was to determine the 

 horizon of the volcanic rocks, the upper slates and shales were not 

 examined in detail ; but one further point may be mentioned. 

 There are shown on the 1-inch Geological Survey map two narrow 

 outcrops of ' felspathic trap,' occurring about a mile west of the 

 main mass and striking in the same direction. These were visited 

 by one of us (S. W. W.), who found that they were in reality 

 quartzose conglomerates absolutely unlike anything seen in the 

 main mass. Possibly the occurrence of white specks of quartz 

 caused these to be confused with the Upper Ashes, which some- 

 times show white specks due to angular fragments of felspar or 

 rhyolite. There seem to be several bands of conglomerate ; judging 

 by surface-features, the principal one extends both north-north- 

 eastwards and south-south-westwards. It must occupy a position 

 many hundreds, or perhaps thousands, of feet above the Upper 

 Ashes. 



There is a little evidence for considering this main conglomerate 

 as the base of the Valentian. It agrees in character with the 

 Cerig-Gwynion Grit of Rhaj^ader 1 on the north-north-east, and 

 with the Shon-Nicholas Conglomerate and Pen-y-ddinas Grit at 

 Llansawel on the south-west. The two last-named grits have 

 already been correlated with the Cerig-Gwynion Grit by Miss H. 

 Drew & Miss I. L. Slater. 2 The outcrop of the grit west of 

 Llanwrtyd, if prolonged, would pass into that of the Cerig-Gwynion 

 Grit of Rhayader and of the grits of the Llansawel district, and 

 surface-features tend to show that such a prolongation does 

 actually occur. 



1 H. Lapworth, ' The Silurian Sequence of Rhayader ' Q. J. G. S. vol. lvi 

 '(1900) p. 95. 



2 ' Notes on the Geology of the District around Llansawel (Carmarthen- 

 shire) ' Ibid. vol. lxvi (1910) p. 402. 



