404 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OE 



2. The Arans. — The ashes lying below the felstonesof Aran Ben- 

 lynn are very unmistakable and quite similar to the coarse and 

 bedded ashes of Cumberland. In the specimen now to be described, 

 the fragments, which are small, are clearly discernible upon the 

 smooth surface from which the slice has been cut, and the rock 

 presents a mottled and somewhat felstone-like appearance. Its 

 microscopic structure is shown in fig. 22. One of the fragments in 

 the slice is of trap, with acicular prisms and triclinic felspar ; but 

 most of them seem to be of felstone containing entire or fragmentary 

 crystals of felspar, several of which are triclinic. In the fine part 

 represented in the figure, many broken crystals are scattered in a 

 hazy base, which presents the felsitic reaction under crossed Nicols. 



In some parts north of Aran Benlynn, and again at the top of 

 Aran Mowddwy, there occur, within the mass of the felstone, brec- 

 ciated and ashy parts. Such intercalated ashes are much altered, 

 and frequently reveal their structure only by weathering on the 

 outside. A slice of a well-bedded but highly altered fine ash on 

 the top of Aran Mowddwy, presents, in every respect, a similar 

 appearance under the microscope to the neighbouring felstone, as 

 shown in fig. 16, except that in the fine ash there are no crystals of 

 felspar or large fragments of such crystals. 



3. The Arenigs. — In some parts of the ash on Y "Wenalt, south of 

 Efridd Trawscoed, there are short irreguular black lines, giving very 

 much the appearance of a "viscous flow. A microscopic section of 

 such a rock reveals clearly that there has been more or less of a flow 

 around the small and larger fragments of which the ash is made up. 



This structure, in the present instance, seems as if it might well 

 be due to subsequent alteration of the beds ; for northwards, where 

 they very much thicken out, they are altered into a compact felstone- 

 like rock, with fragments barely or not at all discernible, except fre- 

 quently on the outside of weathered masses. Prof. Eamsay thus 

 describes them around Llyn Arenig : — " They are thick-bedded, and 

 frequently porphyritic, but so massive, and they so much resemble 

 some of the true felstone porphyries, that the observer is apt to doubt 

 of their truly stratified character, especially as it frequently happens 

 that they are so much jointed that in places they look almost colum- 

 nar. Still, when viewed in favouring lights on a large scale, the 

 long lines of massive beds that streak the steep flanks of the hills 

 generally become sufficiently apparent ; and when traced along the 

 strike to Mynydd JSTodol and Arenig-bach, all doubt ceases ; for, by 

 degrees becoming less jointed and massive, their true-bedded struc- 

 ture comes out with perfect distinctness"*. 



"When such rocks as these are viewed in thin slices under the mi- 

 croscope, it is only possible on some few parts to detect any of the 

 outlines of the original fragments, the general appearance being that 

 so characteristic of the undoubted felstones. Care, of course, is taken 

 to select specimens which seem from their weathered exterior to be 

 made up of comparatively small fragments, so that several or many 

 may come into one field of view. 



* rt The Geology of North Wales," Mern. Geol. Survey, vol. iii. p. 50. 



