H. HICKS ON THE PRE-CAMBRIAN ROCKS OF ST. DAVID**. 233 



bituminous-looking stains; but nothing definitely fossiliferous in 

 character has yet been found in them. 



It is evident that the beds in this section at Porthlisky are the 

 highest of the series to be found in this neighbourhood ; for the dip 

 is invariably towards the south-west in at least three fourths of the 

 whole ridge. The only place where I have yet found them dipping in 

 the opposite direction is at the north-east end of the ridge. The 

 faults north and south of St. David's do not seem to have caused a 

 repetition of the same rocks, but a slight down-throw to the south- 

 east only. It is evident, therefore, that the thickness of the series 

 must be enormous ; but at present I only estimate it at about 15,000 

 feet. Section III. might have been continued out as far as the 

 Bishop's rock, since at low water I have walked to the Crow rock, 

 and have found the beds still dipping in the same direction. The 

 dip throughout is also high, being generally about 80°. In this 

 section there are several dykes of igneous rock, which appear to have 



I. 



A greyish-white sparry mass, with a moderate amount of greenish silicate 

 enclosed. Specks of metallic sulphides, chiefly galena and pyrites. Hard and 

 compact. Sp. gr. 2-801. 



Omitting small quantities, the following is the composition: — 



Portion soluble in hydrochloric acid. 



Water 2"2() 



Alumina (including dissolved silica and 



slight traces of phosphate) 240 



Ferrous oxide (traces of ferric) 1*30 



Lime 2090 



Magnesia 12-50 



Carbonic acid 28 20 



— 07-50 



Portion insoluble in hydrochloric acid. 



Silica 26-00 



Alumina (traces of iron oxide) 3L0 



Lime 1 "25 



Magnesia L00 



Soda and potash L15 



3250 



10000 



The bulk of the mass therefore consists of sparry carbonates, viz. crystalline 

 dolomite, with some calcite. It is probable that there is also a small quantity 

 of amorphous carbonate of lime. In the soluble portion there is a small excess 

 of lime and magnesia beyond what is necessary to satisfy the carbonic acid; 

 this, together with some of the alumina and a portion of the ferrous oxide, is 

 in combination with part of the silica. Some of it goes to form the serpenti- 

 nous or chloritic mineral which imparts a greenish tinge to the mass. In the 

 insoluble portion of the analysis, the abundance of silica points to the presence 

 of free quartz, which, indeed, may be recognized in the hand-specimen ; and there is 

 also probably some fine felspathic debris, partly kaolinized, which would account 

 for much of the alumina of this part of the analysis. The insoluble lime and mag- 



