PRE-CAMBRIAN ROCKS OF ST. DAVID S. 



269 



pect precision or accuracy in the determination of rocks then made, 

 either in the Survey or out of it. The rock of St. David's was 

 marked down as " Syenite" on the Survey Map and Section ; and so 

 it remains up to this hour. But in the present discussion the main 

 fact to be noticed in regard to the Survey mapping is that the rock 

 in question was declared to be an eruptive rock, intrusive in, and 

 therefore later than, Cambrian strata. 



In his latest papers Dr. Hicks thus describes the rock in question. 

 "The rocks included under this name [Dimetian] .... everywhere 

 show more or less distinct lines of bedding, striking from about N.W. 

 to S.E. They vary also, to some extent, in their mineralogical char- 

 acters, if examined at different points, and have apparently an 

 order of succession in which these changes occur at recognisable 

 horizons. The prevailing rocks in this group are of a granitoid 

 character, usually of rather a massive, but sometimes of a schistose 

 nature. 



" Sometimes almost pure quartzites are found; but by far the 

 largest proportion contain an admixture of felspar of a white or 

 pinkish colour. Specks of viridite usually occur more or less through- 

 out, and sometimes give quite a tinge to the rock. Mica occurs 

 sparingly, for the most part; but there are occasionally micaceous, 

 chloritic, impure limestone and serpentinous bands. At some places 

 also, thin, compact, white bands of a more highly felspathic char- 

 acter occur. Some of the beds assume a brecciated appearance, the 

 masses being generally angular or subangular, and in composition 

 much like the associated rocks. 



" Speaking generally, the majority of the rocks comprising this 

 group are highly quartzose, of a granitoid or rather massively gneissic 

 nature, and usually easily recognisable by these characters ; their 

 strike is about from N.W. to S.E."*. 



" This formation [Dimetian] consists chiefly of highly crystalline 

 gneissic rocks, the prevailing types, however, being the so-called 

 granitoid rocks, made up largely of quartz with some pinkish or 

 white felspar. Hornblende is much less abundant than in the 

 Lewisian rocks ; but mica is more frequently met with. It may be 

 said to consist chiefly of acid types of rocks, whilst the former is 

 made up mainly of basic types. Bands of limestone, hornblende, 

 chlorite and micaceous schists occur occasionally in this formation "f. 



These rocks are regarded by Dr. Hicks as a great, bedded, meta- 

 morphic Pre-Cambrian series, later in age than the ancient gneiss 

 of the Hebrides. They are subdivided by him into two groups — a 

 lower, consisting of " the massive granitoid and gneissose rocks of 

 Bryn-y-Garn, St. Davids," and an upper, composed of " the so-called 

 quartz-schists of Porth-lisky " $. 



* Proc. G-eol. Assoc, vol. vii. no. i. p. 61 (1881). On the next page the " Di- 

 metian" rocks are asserted to be " chiefly of clastic origin." 



t Popular Science Eeview, N. S. vol. v. p. 291 (1881). It is to be noted that 

 this paragraph relates to "Dimetian" rocks generally, and is not intended 

 to apply specially to those of St. David's. 



\ Popular Science Eeview, loc. cit. 



