412 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



6. Great Gable, Wastdale-Eead. — This example is one of a 

 very highly altered, finely bedded ash, the bedding being plainly 

 visible on the weathered face of the cliff, but the rock in its inte- 

 rior "presenting an exceedingly compact, blue, felstone-like appear- 

 ance. 



Figure 14 gives the structure as seen with a high power — a 

 very finely granular base, with numerous disseminated grains of chlo- 

 rite and patches of closely aggregated particles. "Were it not for the 

 chloritic grains and some few scattered broken crystals, there would 

 be nothing whatever to distinguish this base from the altered slate 

 of Craig Wen. Under crossed Nicols, the ground is dark, with bright, 

 coloured, spangled points of light, due partly to the chlorite. Besides 

 some broken acicular crystals, such as that in the figure, there seem 

 to be some grains of augite (?) partially altered. In parts of the base, 

 where the granules are not so thickly scattered, it presents a hazy 

 felstone-like appearance, and with crossed prisms shows the felsitic 

 reaction. 



7. Bigghause End, Vale of St. John. — The last of this series now 

 to be described is a very compact rock, weathering exceedingly like 

 some of the Welsh felstones, but passing almost imperceptibly into 

 clearly marked felspathic ashes. 



Figure 16, illustrating the structure of the Aran Mowddwy fel- 

 stone, perfectly illustrates that of this rock also. The base consists 

 of patches and particles of chlorite on a hazy white ground, a com- 

 pound of quartz and felspar. Under crossed Nicols the coloured- 

 breccia reaction is very well marked, as shown in fig. 15. There are 

 a few fragments of felspar, some triclinic. 



c. Manner of metamorphism. — In all these examples of altered 

 ash, while their appearance in hand-specimens is often extremely 

 like that of felstones, their general aspect and deportment in the 

 field is such as led me, previous to any microscopic examination, to 

 map them as altered beds of fragmentary material. Either traces 

 of fragments or bedding are everywhere discernible, or the rocks 

 pass gradually into less and less altered and unmistakable ash. 

 The fullest development of these, at first sight, somewhat puzzling 

 beds, is in the district north of and around Scawfell ; in fact they 

 increase in extent as the great tract of Eskdale granite is 

 approached. Now it is clear that some kind of alteration might be 

 expected all around the granitic centres of Eskdale, Wastdale, and 

 Ennerdale; and since the rocks immediately surrounding these 

 deeply formed igneous masses are for the most part composed of 

 felspathic ashes, it is not surprising to find that they have become 

 altered into rocks much resembling true felstones. Sorby has shown 

 (Brit. Assoc. Eep. 1857, p. 92) that the material of both quartz and 

 mica might be derived from felspar ; so that felstone (a mixture of 

 felspar and quartz) may well be formed by the metamorphism of 

 felspathic ashes. At any rate it seems probable that the alteration 

 has, in many cases, been sufficient to cause the development of a 

 great quantity of granular chloritic matter, often flowing around the 

 larger fragments, giving rise to the streaky appearance so similar to 



