335 



structures, and sometimes also micro-pegmatitic structures. These rocks 

 are regarded by Dr. GEIKIE as forming the peripheral portion of the 

 granite. They have been described by Messrs. TAWNEY^and I)AviES/ 2 > 

 by Dr. GEIKIE < 3 > and Professor BLAKE. (4) 



Professor BLAKE distinguishes two varieties, the "spotted felsite" 

 and the quartz-felsite. The porphyritic crystals of the former are 

 mainly felspar. The ground-mass is minutely spherulitic or micro- 

 crystalline. The spherulites have, as a rule, a well-defined periphery 

 and give a fixed black cross. They are independent of the porphyritic 

 constituents (felspar and quartz) so far as their distribution is concerned. 

 The porphyritic crystals of the latter are both quartz and felspar (often 

 striated) ; the spherulites are large, without a well-defined black cross, 

 devoid of sharp outlines and grouped round the porphyritic consti- 

 tuents. The quartz occurs in the corroded forms characteristic of the 

 felsites and liparites. The matrix in which the porphyritic consti- 

 tuents with their spherulitic appendages lie is micro-crystalline. Epi- 

 dote is common in both varieties, and often occurs in radiating sheaves. 

 It is an unfortunate circumstance that there is a difference of opinion 

 as to the geological relations of the rocks above described. Dr. GEIKIE 

 regards them as the apophyses of the granite; Dr. HICKS on the other 

 hand speaks of them as rhyolitic lavas. Both observers agree, however, 

 that there are fragmental rocks of acid composition in the St. David's 

 area, and Dr. GEIKIE gives the analyses of three rocks which he classes 

 as felsitic tuffs. 



I. II. in. 



SiOo 80-59 ... 73-42 ... 72-63 



ALA 11-29 ... 12-09 ... 16-23 



FeA 0-28 ... 0-91 ... 2-70 



FeO 1-41 ... 3-13 ... 0-48 



MnO tr. ... 0-25 



CaO 0-52 ... 2-94 ... 0-18 



MgO 0-95 ... 1-12 ... 1-36 



K.O 2-98 ... 1-67 ... 3-35 



Na.0 0-72 ... 3-88 ... 0-15 



Loss 1-96 ... 1-28 ... 3-04 



100-70 100-69 100-12 



Sp. Gr. 2-55 ... 2'74 



I. Greenish felsitic breccia, Clcgyr Hill. Rock composed of angular fragments of various 

 felsites embedded in a greenish matrix. Analysis by Mr. WILSON. 



II. Grey felsitic tuff, north of the bridge over the Allan river, north from the Schools, 

 St. David's. Analysis by Mr. WILSON. 



III. Pale pinkish-white, finely schistose tuff, a characteristic sample of the " Porth-lisky 

 schists." Analysis by M. RENARD. 



(1) Proc. Bristol Naturalists' Soc., Vol. II., Part 2. 



(2) Q.J.G.S., Vol. XXXIV. (1878), p. 164. 



(3) Q.J.G.S., Vol. XXXIX. (1883), p. 315. 



(4) Q.J.G.S., Vol. XL. (1884), p. 303. 



