PKE-CAMBRIAN ROCKS OF ST. DAVID'S. 299 



of diabase may be detected ; but I could find no trace of pieces of the 

 peculiar micro-crystalline spherulitic quartz porphyries of St. David's. 

 There is but little that could be called matrix cementing the lapilli 

 together. The presence of fragments of diabase may possibly reduce 

 the proportion of silica and increase that of magnesia, as compared 

 with what would otherwise have been present in the rock. 



Some of the tuffs appear to have been a kind of volcanic mud. -A 

 specimen of this nature collected from the road-side section, north 

 of the Board School, presents a finely granular paste enclosing 

 abundant angular and subangular lapilli of diabase, a smaller pro- 

 portion of felsite (sometimes displaying perfect fluxion-structure), 

 broken plagioclase crystals, and a greenish micaceous mineral which 

 has been subsequently developed out of the matrix between the 

 lapilli. 



Though they lie above the main volcanic group, I may refer here 

 to the thin bands of tuff at Castell, of which, from their interest in 

 relation to the true Cambrian age of the volcanic group, I have had a 

 number of slices made for microscopic investigation (Plate IX. fig. 3). 

 They are not quite so fresh as the tuff that occurs in thicker masses ; 

 but their volcanic origin is readily observable. One band appears 

 to be made up of the debris of some basic rock like the diabase of 

 the district, through which detached plagioclase crystals are scattered. 

 The lapilli are subangular; and around their border a granular deposit 

 of haematite has taken place, giving a red colour to the rock. 

 Another band presents small angular lapilli, almost entirely composed 

 of a substance which to the naked eye, or with a lens, is dull, white, 

 and clay-like, easily scratched, and slightly unctuous to the touch. 

 Under the microscope, with a low power, it becomes pale greyish 

 green and transparent, and is seen to consist in large part of altered 

 felspar crystals, partially kaolinized and partially changed into 

 white mica and calcite. These scattered crystals are true volcanic 

 lapilli. and have not been derived from the mechanical waste of 

 any preexisting volcanic rock. In the tuffs interstratified with the 

 conglomerate, at the quarry above Porth-clais, though much decom- 

 posed, crystals of plagioclase can likewise still be traced. These beds 

 are also true tuffs, and not mere detritus due to mechanical degra- 

 dation. 



The general result of this study of the microscopic structure of 

 the tuffs may be briefly summed up as follows : — 



1. They are almost wholly composed of fragments of eruptive 

 rocks, sometimes rounded, but usually angular or subangular. In 

 the more granular varieties very little matrix is present; it con- 

 sists of fine debris of the same materials. jNo detached microliths 

 occur, such as are common among modern volcanic ashes; but there 

 are abundant ejected crystals. In these respects the Cambrian tuffs 

 are quite like those of the other Palaeozoic systems. A mingling 

 of grains of quartz-sand may indicate the intermixture of ordinary 

 with volcanic sediment. 



2. They may be divided into two groups — one composed mainly 

 of fragments of diabase or other similar basic rocks, the other of 



