PEE-CAMBRIAN ROCKS OF 8T. DAVID'S. 271 



been so confidently and constantly affirmed that, for a time, we were 

 inclined to believe we had missed the proper exposures. It was not 

 nntil we had diligently hammered every knob and boss of rock in the 

 whole district, without discovering any other structure than that of 

 an eruptive mass, that we were driven to abandon as entirely imagi- 

 nary the idea of a bedded structure and metamorphic origin for 

 the central rock of the ridge. The comparatively limited and dis- 

 connected sections of the interior amply suffice to make this quite 

 clear. But the admirable continuous sections of the coast-line reveal 

 the structure of the rock so completely that one could not but ask 

 oneself the question many times a day, how such a rock could ever, 

 by any possible stretch of the imagination, be credited with a bedded 

 structure and a metamorphic origin. Cut at all angles by the sinu- 

 osities of the coast-line, it can be studied foot by foot across its 

 entire breadth. Did it, therefore, possess foliation of any kind, or 

 were it made up of parallel bands of different lithological characters, 

 such a structure could not possibly escape notice. After the most 

 careful search, however, neither my companion nor myself could 

 discover any thing of the sort. But for the published statements 

 regarding the rock, we should never have thought of making any 

 such search ; for the first few exposures would have sufficed to mark 

 it out as unquestionably an eruptive mass. 



The petrographical characters of this rock will be given in Part II. 

 of this paper (p. 313). To the naked eye it appears everywhere to 

 be thoroughly crystalline and granitic in structure, like a granite of 

 medium grain, perfectly amorphous, without any trace of ground- 

 mass or any approach to foliation. It can readily be seen to be 

 composed mainly of a granular crystalline aggregate of quartz and 

 felspar with abundant minute black or dark-green specks, which, by 

 their decomposition, give rise to a diffused greenish discoloration. 

 These dark specks were regarded by the Geological Survey as horn- 

 blende; and hence, according to the old nomenclature, the rock was 

 termed a syenite. On the other hand, were these dark green specks 

 showu to be a mica, there could be no hesitation in classing the 

 rock as a variety of granite. Whether examined in mass, in hand- 

 specimens, or under the microscope, it presents the ordinary struc- 

 ture of a granite. I shall therefore speak of it simply as a granite, 

 and leave its peculiarities of composition to be discussed in the sequel. 

 I may here mention, in passing, that I have examined microscopi- 

 cally a large series of slices of the rocks of St. David's, and that the 

 result of this examination will be given in Part II. (as previously 

 stated). 



The numerous rocky bosses upon the ridge south-west from St. 

 David's, and still more the long coast-section from Porth-lisky 

 eastward, everywhere present a massive rock entirely destitute of 

 definite structure, but traversed by irregular joints, which divide 

 it into blocks, as in any ordinary granite. Here and there by a 

 dominant set of joints it is separated into rudely parallel beds, or 

 even thin slabs, as at Porth-lisky and eastward, where one series 

 of joints, running from N.N.E. to S.S.W. with a high inclination, 



