234 H. HICKS ON THE pre-cambrian rocks of si. david's. 



been intruded into the series before they were elevated out of the 

 horizontal position, and before metamorphism had taken place; they 

 are probably fine-grained altered dolerites, being columnar in struc- 

 ture and the columns at right angles to the plane of the dyke*. 



nesia form part of some pyroxenic matter, which is evidently distributed through- 

 out the mass. 



II. 



A greyish green rock with a gritty feel and very uneven fracture ; is pervaded 

 by much brown-coloured spar. Sp. gr. of particular fragment 2-774. No me- 

 tallic sulphides noticeable. 



As in the previous case, omitting smaller quantities, the following is the 

 composition :— 



Portion soluble in sulphuric acid. 



Water 350 



Alumina (very slight traces of phosphate) 4*10 



Ferrous oxide (traces of manganese) 609 



Ferric oxide 250 



Lime 2060 



Magnesia 6*70 



Carbonic acid 20*00 



Dissolved silica 3*10 



67-00 



Insoluble portion (principally silica) 33*00 



100-00 



The insoluble portion was not estimated in this case ; but the same fibrous 

 pyroxenic mineral may be seen, whence there can be little doubt that small 

 quantities of lime and magnesia would be found. 



There is less sparry carbonate in this than in No. I. ; indeed much of the 

 carbonate of lime would seem to be amorphous. The brown spar is a ferriferous 

 dolomite, which in some specimens exhibits the curved cleavage of that mineral 

 very distinctly. The rest of the soluble magnesia goes to form the chloritic 

 matter, which is uniformly disseminated, and which, along with the quartz, and 

 probably some fine felspathic debris, constitutes a large portion of the rock. 



However we might hesitate about No. I., there can be very little doubt that No. 

 II. has all the appearances of having been a mechanical aggregate. 



The following is a description of some sections of this dolomitic rock, by Mr. 

 T. Davies, F.G.S : — " In thin sections the principal mass of the rock is seen to 

 consist of a crystalline rhombohedral carbonate (either calcite or a dolomite), 

 varying somewhat in texture. The numerous interstices and cavities have been 

 lined with distinct rhombohedral crystals of the mineral constituting the mass. 

 These interspaces have subsequently been filled up with quartz, in a few in- 

 stances accompanied by clear calcite, easily distinguishable from the rock- 

 mass. The rock is but semitranslucent when prepared very thin, which is 

 owing to the presence of a grey opaque minutely divided substance, which per- 

 vades pretty evenly the whole mass ; its nature is entirely unrecognizable by 

 the microscope. The quartz encloses, in cavities and minutely diffused, a con- 

 siderable amount of a fine-grained granular material, probably of the same 

 nature as that pervading the remainder of the rock : some of the cavities also 

 contain a structureless substance, apparently a chloritic mineral, varying in 

 colour from a grass-green to grey." 



* In general appearance these altered dolerites are scarcely distinguishable at 



