672 MR. C. A. MATLEY ON THE [-^-Ug. 1 899,. 



stages in the breaking-up of minute grit-bands to form tiny ' pebbles/ 

 We can pass from the microscopic slide to the hand-specimen, from 

 the hand-specimen to the coast-exposure or quarry, and see the 

 same stages of disruption of the harder rocks among the softer, on 

 an ever-increasing scale. We have even gone farther and found 

 that the isolated quartz-knobs and the detached limestane-masses 

 are separated portions of great rock-sheets, and it is a matter of 

 definition rather than a question of principle whether they should 

 be called the ' pebbles ' of a great crush-conglomerate. 



(c) The Ordovician Rocks. 



Prof. Blake first recognized the Ordovician age of the Ogof 

 Gynfor conglomerates ; the fossils recorded on p. 640 confirm his 

 identification and fix the age as Llandeilo. These pass up, as 

 shown in the section (fig. 3, p. 648), into black shales, which are 

 so quartzose as almost to merit the name of grits; their upward 

 continuation seems to be the I)icranograj)tics-sh.ales of Penterfyn, 

 exposed inland, which contain some fiaggy bands of oolitic iron- 

 stone. The presence of Ordovician ironstones in Anglesey hsiS, ipso 

 facto, been considered to suggest an Arenig horizon,^ but the 

 Penterfyn ironstone is distinctly Llandeilo. The boundaries of this 

 Ordovician tract are the two main faults shown in fig. 3, p. 648, 

 which give a gradually widening outcrop till the series is cut off by 

 an east-and-west fault from Perth Padrig ; but these faults enclose 

 much quartzite as well as Ordovician beds. Another tract, of a 

 V-shape, mainly of conglomerates and grits, lies between this area 

 and Perth Llanlliana. Its structure is obscured by cleavage, but 

 the beds appear to be much folded and faulted, as the accompanying 

 sketch (fig. 13, p. 673) indicates. 



We pass now to the noble headland of Llanlliana or Dinas Cynfor, 

 whose fine cliffs are mostly composed of Ordovician conglomerates. 

 The uppermost beds are black-banded, grey, quartzose shales, 

 cleaved into rude slates, and we recognize their equivalence with 

 the darker gritty shale which occupies a similar position at Ogof 

 Gynfor. South of these conglomerates and slates is the reddish- 

 purple conglomerate that lies on the quartzite. This is so cut off 

 from the other conglomerates that it is a question whether it should 

 be included in the Ordovician ; Blake was doubtful whether it should, 

 and its colour is characteristic of the old rocks. But it seems to be 

 partly derived from the underlying quartzite, its colour is sometimes 

 leached out and then it closely resembles the other conglomerates, 

 and fig. 13 has shown us that the undoubted Ordovician conglo- 

 merates contain purple zones. We may, therefore, for the present 

 regard it as the basal bed of the Ordovician, and we can follow it 

 from Hell's Mouth to Porth Wen still lying on the quartzite. The 

 folded and cleaved conglomerates and grits, which lie between the 

 purple rock and the sea, and form the headland of Torllwyn now 



* Quart. Journ. Geol. Soc. vol, xxxviii (1882) p. 21. 



