﻿626 ME. W. G. FEAENSIDES ON THE GEOLOGY OP [Aug. I905, 



The Lower or Andesitic Ashes of the Upper Series [5]. 



As above remarked, the Upper Daerfawr Shales merge imper- 

 ceptibly into the Lowest Ashes of the Upper Series. This division 

 is more basic than the rest of the series, and generally includes 

 rather abundant shale-fragments. It is often agglomeratic (4672- 

 73) and well bedded, and may include several thin lava-flows, but 

 none of these extend over emj considerable area. In general, it is 

 less resistant than the Middle or Massive Series, and, being well 

 jointed, usually forms the steep faces of cliffs and precipices which 

 are capped by that series. 



The Middle or Massive Ashes of the Upper Series [4] . 



The Middle division of the Upper Series consists of ashes, which 

 are, on the whole, much finer and more altered than those either 

 above or below. They are, however, almost devoid of bedding, and, 

 being jointed on an extremely-large scale, weather out into very 

 massive and imposing forms. In the upper part they have taken on 

 a sort of incipient cleavage-jointing, and consequently often deter- 

 mine rough and almost pinnacular crags. In lower ground they 

 weather most curiously to a greenish cream-coloured rock, with 

 abundant streaks and irregular veins of some pale chloritic mineral 

 which suggests pinite. Concurrently witli this form of alteration 

 there seems to be a great tendency to segregation and oxidation 

 of the manganese which is present throughout the rock, and in 

 former days manganese was worked, or prospected for, at frequent 

 intervals all over the outcrop. The manganese obtained was in 

 the form of botryoidal pyrolusite, and it is always said that the ore 

 invariably dies out in depth. Chemically the rocks of this series are 

 intermediate between the acid andesites of the Lower, and the basic 

 rhyolites of the Upper, group. They are closely similar to many of 

 the felsites and felsitic ashes of Snowdonia. 



The Upper or Rhyolitic Ashes of the Upper Series [3]. 



The Upper division is even more vaguely defined than the others. 

 It consists of ashes more distinctly bedded than those below, and its 

 members usually contain recognizable chips of felspar-crystals. The 

 highest members of the series become so well bedded that they 

 appear almost like massive flags in their weathering. The lower 

 part of this Upper division, and, to some extent, the upper part of 

 the Middle also are frequently almost impregnated with ramifying 

 quartz-veins. In other places apparently-similar beds have been 

 silicified along irregular veins, until they appear like cherts or 

 adinoles (4674). For a long time I was unable to understand the 

 nature of the alteration ; a series of silica-determinations, however, 

 at once set the matter at rest, and showed conclusively that in the 

 more basic members of the Ash Series chert-like veins take the 

 place of the quartz- veins of the acid members. The same series of 



