232 H. HICKS ON THE rBE-CAMBBIAN BOCKS 01-' ST. DAVID S. 



the character of the beds which are associated with them. Instead 

 of the ashy shales as at St. David's, we now find massive beds of cal- 

 careous shale and chloritic schists, and associated with them also 

 dolomitic limestone beds several feet in thickness. The limestone 

 beds I only discovered for the first time last year. None of the 

 shales at St. David's contain much calcareous matter, and they do 

 not effervesce with acid ; but these shales and schists at Torthlisky, 

 including some of the more compact quartzose schists, effervesce 

 most freely, and on analysis show the presence of frequently over 

 20 per cent, of carbonates. The rocks are sometimes flaggy, 

 and split easily in the line of the bedding ; but usually they are more 

 schistose in nature. The beds vary in thickness from 2 to 15 

 feet. In addition to the carbonate of lime in these beds, I have 

 detected in them about 0*5 per cent, of phosphoric acid ; hence 

 it is more than probable that the lime was deposited by organic 

 aid. 



I have not as yet discovered any thing in these beds of a decidedly 

 organic nature. There are markings on some of the surfaces where 

 the rock is split along the line of bedding, which may possibly be 

 organic ; but at present, and until further specimens have been found, 

 I would speak with doubt as to their nature. 



The quartz schists have generally a greenish or a purplish tint, 

 and the surfaces of the bedding and joints a soft greasy feel. 



In this section there are five distinct bands of impure limestone, 

 separated from each other by compact beds of quartz schists. They 

 vary in thickness from 1 to 3 feet, and show considerable differences 

 in their composition. The three lower bands are fine-grained and 

 compact in texture, of a greyish colour, and contain occasionally 

 scattered bits of quartz, serpentine, pyrites, galena, &c. The two 

 upper bands are darker and more ferruginous in character, and 

 usually contain more serpentine*. Occasionally in both are seen 



"It consists of quartz, orthoclase, and an anortbic felspar, with small specks of 

 chlorite; the quarlz, however, forms by far the greater part, and is intercrys- 

 tallized with the felspar in a peculiar manner, somewhat resembling that of 

 graphic granite on a very small scale, recalling to mind very forcibly the 

 mtergrowth (if it be such) of the felspar and quartz in the Mull granites. 

 Both of the felspars appear to be considerably altered — so much so that, except 

 in a few instances, the two felspars are not distinguishable from each other. 

 To me its position would appear to be among the metamorphic crystalline 

 schists. Further examination of the rock-specimens, although parts of the 

 same mas?, show great vai'iety in the proportions of the constituents. In all 

 of them quartz is by far the predominating mineral ; indeed some of them con- 

 sist almost entirely of it. though it varies from medium crystalline to crypto- 

 crystalline in texture. The chlorite is present in them all, and appears to vary 

 less in amount than the other minerals. In some no calcite or dolomite is 

 visible, whilst in others both occur in large quantity. Only one of the speci- 

 mens shows undoubted crystals of a felspar. It is evident that no two analyses 

 or microscopic examinations of fragments of such a rock, taken from a few feet 

 apart, could agree." 



* The following are analvses of two specimens of these rocks, bv Mr. \V. II. 

 Hudleston, M.A., F.C.S. :— 



