part 3] LOWER PAL.EOZOIC OF ABTHOG-DOLGELLEY. 283 



that they show textures intermediate between those of the lavas and the 

 normal diabases. 



The mass which forms Tyddyn Camp Hill and is intrusive into the Crogenen 

 Slates seems to belong to this series. It is a fine-grained rock, the texture 

 being but little coarser than that of a typical spilite. Like a spilite, it 

 consists [C 244] of a plexus of minute felspar-laths and fibres often curved 

 and forked, the interspaces being occupied by pale-green chlorite and wisps 

 of hornblende, along with patches of murky interstitial material which 

 represents altered glass. Vesicles infilled with chlorite and epidote are 

 abundant. Felspar-phenocrysts occur sparsely ; their outlines are irregular, 

 ■owing to corrosion. There can be no doubt that the rock has practically the 

 •composition of a spilite. 



(B) The Granophyres and Related Rocks. 



(a) Field - relations. — Although intrusions of basic material 

 .are so abundant, only two intrusions of thoroughly acid material 

 .are known in the whole district round Cader Idris. Each of 

 these is, however, of considerable size, so that the bulk of acid 

 material exceeds the total bulk of the far more numerous basic 

 intrusions. Of the two acid intrusions, one is well known from 

 its forming so prominent a feature in the cliffs of Cader Idris ; 

 it has been described by Ramsay 1 and by Prof. Gr. A. J. Cole & 

 Mr. A. V. Jennings " under the terms ' felspar-porphyry ' and 

 ' eurite ' respectively. The other intrusion — the Crogenen grano- 

 phyre — has received but passing notice from previous observers. 



Actually, we find a whole set of intrusions made up of grano- 

 pli3 r ric rocks which, considered together, show all transitions 

 from thoroughly basic to thoroughly acid rocks. The more basic 

 types approximate to quartz-dolerites in composition, and they 

 pass through types which may be described as relatively basic 

 granophyres, or ' markfieldites,' into normal acid granophyres 

 which have the composition of a microgranite. Acid granoph3 r re 

 is the dominant type as regards bulk, the more basic types merely 

 representing early differentiates from the acid magma. In the 

 field the most basic of these granophyric differentiates are at first 

 sight liable to be confused with the diabases. But a closer inspection 

 shows that they should be linked with the acid granophyres, and 

 that they belong to a set of intrusions distinct from, and of later 

 date than, the true ophitic diabases. We do not preclude the 

 possibility of there being a genetic connexion between the ophitic 

 diabases and the acid granophyres ; that some such connexion exists 

 is almost certain. But we wish to emphasize that the basic rocks 

 in question are more closely linked to the acid granophyres than to 

 the ophitic diabases. Within the area mapped granophyric rocks 

 occur in five intrusions, which are all laccolitic or sill-like in form. 

 Four of these are relatively small, and are composed either entirely 

 or in part of more or less basic material. In general, each laccolite 

 or sill is less basic than the one stratigraphically below it ; further, 

 within the limits of each mass the rocks at the bottom tend to be 



1 ' Geology of North Wales ' Mem. Geol. Surv. vol. iii, 1st ed. (1866) p. 27. 



2 Q. J. G. S. vol. xlv (1889) p. 433. 



