410 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OP 



where much of the felspathic ash is a good deal altered. In appear- 

 ance, the rock under examination is mottled, of a light blue or 

 greyish colour, and very similar to parts of the Lower Aran Ash 

 already described (fig. 22). Its general microscopic structure is 

 represented in fig. 9, where an attempt is made to show the mi- 

 nutely granular character of the base, precisely similar to that of the 

 two test-specimens already described. There are many fragments of 

 felspar crystals scattered about, and one or two tolerably perfect, 

 such as the orthoclase twin shown in the figure. Both plagioclase 

 and orthoclase occur, very highly altered, and frequently more so 

 within their interior than round their edges. There is a good deal 

 of chlorite, which shows its dichroism very markedly in some cases, 

 and occurs disseminated throughout the base and filling cracks and 

 cavities. Some of the green pseudomorphs may be after augite ; but 

 there is none of the unaltered mineral left, nor are there present any 

 acicular crystals or magnetite grains. Prom the above description 

 it will be evident that this rock presents very decided differences 

 from the undoubted lavas of the district : more approximates to the 

 Welsh felstones ; but most resembles in its structure the test-speci- 

 men of ash already described. When the slide represented in fig. 9 

 is viewed with polarized light, under crossed prisms, the appearance 

 is similar to that shown in fig. 10 — altered felspar crystals and frag- 

 ments on a dark ground with scattered points of light. 



4. Hart Side, north of Glencoindale Head. — This example is one 

 of a coarse ash. It has the same general mottled felsitic appearance 

 as the last, but on the weathered exterior and upon a smoothly cut 

 surface the separate fragments are clearly discernible. The base, 

 between the fragments, is made up of minute particles, as in the 

 former examples ; but in this case there would seem to have been a 

 decided flow of the granular matrix around- the imbedded pieces 

 (fig. 13). The question arises, when did this flow take place? Now 

 along the lines of flow, scattered throughout the base, and closely 

 surrounding many of the fragments, occurs a considerable quantity 

 of chloritic matter. This is probably a product of alteration ; the 

 way in which it has taken the place of many minerals throughout 

 both these and the trap rocks, and the manner in which it some- 

 times occurs close round the edges of fragments in the ashes, suggest 

 that it has been formed within the rocks perhaps long after their 

 origination. Its occurrence in these flowing lines in a rock which 

 has evidently undergone a large amount of alteration — an alteration 

 which has been sufficient to weld together fragments of considerable 

 size so as to be undistinguishable from each other on a newly fractured 

 surface — would seem to point to the conclusion that the lines of flow 

 were produced in that very process of alteration ; and I do not think 

 that any other conclusion can, with equal probability, be arrived at. 



Among the felsitic fragments there occur some tolerably perfect 

 twin crystals of orthoclase. In a few places, where the granules in 

 the base are not so thickly crowded together, there is, under crossed 

 prisms, more or less of the coloured-breccia reaction characteristic 

 of felstones. 



