374 MESSES. A. V. JENNINGS AND G. J. WILLIAMS 



green-coloured rock, which seems also to be a grit. In this the 

 grains are more angular, there is a larger proportion of felspar, 

 mica, and magnetite, and garnet is also present. 



(c) The Arenig Series. — The beds immediately above the grit do 

 not differ greatly from those below. They still consist of spotted 

 rocks with a flaggy structure. In some cases there is a well- 

 developed " secondary cleavage " or " pillaring " at right angles to 

 the bedding and directed 13° W. of N., as in the slate quarries 

 above. So distinct is it that these beds are quarried when large 

 straight-edged slabs are wanted, blocks of fifteen feet in length 

 being easily obtained. Beds of fine ash also occur in this level, and 

 north of Cwm Orthin they form a considerable local thickening of the 

 strata between the grit and the agglomerates. At this point they 

 appear as a hard, very light- coloured rock, spotted with white, and 

 sometimes finely banded. Though different in appearance from the 

 spotted flags below, this rock has much the same structure, being 

 made up of fine colourless crystalline particles with disseminated 

 chlorite. The white spots might be taken at first for developing 

 crystals, but when they are seen under the microscope this is found 

 not to be the case. 



Above, there seems to be an increasing proportion of clayey 

 material, and a blue colour in the rocks appears, increasing until 

 the first bed of true slate is reached. This, the " Lower Slate 

 Bed " as we propose to call it throughout in this paper, consists of 

 an impure slate, often sandy and ferruginous, which has never as 

 yet paid for working. Trial levels may be seen repeatedly along 

 Moelwyn, as also under Gareg Ddu and Manod Mawr. It has not 

 yielded fossils on the Moelwyn Range, but a Lingula and Orthis 

 Caraiisii have been found in it at Cae Clyd, and a Tetragraptus south- 

 east of Manod Mawr. It is interesting to note that the slate bed is 

 covered in places by a fine-grained, grey rock, similar to the " hards." 

 of the Llandeilo quarries, which we hope to examine in greater detail 

 at some future time. Just at their junction occurs a grit band a 

 few inches thick, full of ovoid black ** pebbles ; " it is very similar to 

 the bed mentioned by Messrs. Cole and Holland * as occurring on 

 Ehobell Fawr, and regarded by them as equivalent to the Garth 

 Grit. It would be rash to say that the two beds are equivalent, 

 especially as we have only observed that on Moelwyn at one spot, 

 but the fact that there are several minor grit bands above the true 

 "Garth Grit" is worth the notice of geologists in North Wales. 

 Another feature of interest is that the black " pebbles " in this bed 

 show under the microscope a number of " oolitic " grains with con- 

 centric structure, recalling at once the appearance of the Arenig 

 ironstone of other localities. 



The igneous rocks of the Arenig series have been regarded as 

 consisting of felstones and agglomerates ; an upper and lower fel- 

 stone alternating with upper and lower agglomerates. Closer ex- 

 amination shows that the succession is by no means uniform in 



* Cole and Holland, Geol. Mag. (1890) p. 449. 



