■300 PEOF. A. H. COX AtfD MR. A. K. WELLS OIS* THE [vol. lxxvi, 



The Devon rock is compared with one at Newly n (Cornwall) 



' [which] has far too much silica and alkalies and too little lime and magnesia 

 to pass as an ordinary diabase. These bring- it much nearer the group of 

 ■augite-andesites and the spilites, with some analyses of which it is closely 

 comparable.' ' 



The markf'ieldite type. — This type occurs in the upper half 

 of the Craig-y-Castell mass, which represents the largest and most 

 typical development. It occurs also in the upper part of the 

 smaller Tyn-y-llwyn sill, and again in the middle of the larger 

 Tyn-y-llwyn mass. Finally, it recurs locally along the upper 

 margin of the last-named intrusion. 



In the field, rocks of this type are characterized by a regular and 

 large-scale, sometimes columnar, jointing, so that scree-material is 

 •composed of great rectangular blocks. These weather grey, and 

 present superficially a deceptive resemblance to the normal diabases 

 •of the district ; but a closer examination reveals their marked 

 granophyric character. The markfieldite type 3 evidently serves to 

 link up the quartz -dolerites with the normal acid granophyres of 

 •euritetype, and every stage of the transition is to be seen. Taking, 

 however, an average specimen of these coarse-grained transitional 

 rocks, one will find it to agree closely both in texture and in com- 

 position with the well-known rocks of Groby and Mark field in 

 Charn wood Forest 3 ; hence the \ise of the term markfieldite. It 

 is well known that the Charnwood-Forest rocks have associated 

 with them, usually as separate intrusions, more basic types which 

 show transitions to quartz-dolerites. 



The characteristic feature of the markfieldites is the presence of 

 abundant, large, idiomorphic felspars, set in a micrographic ground- 

 mass. The granophyric intergrowths are more perfect in this than 

 in any other of the granophyric rocks of the district (PI. XVIII, 

 figs. 4, 5 & 6), a fact which is of some significance in regard to 

 the genesis of the rocks (see below, p. 303). Besides being more 

 perfect, the intergrowth is usually also much coarser than in the 

 more acid or more basic types. The scale of the intergrowth, 

 however, varies considerably, and sinks in some cases to almost 

 ultramicroscopic fineness [C 241] ; such fine-grained intergrowths 

 are more common in the more basic varieties. 



Besides taking part in the intergrowth, quartz is present in large 

 granules which are bigger and also more abundant than those in 

 the quartz-dolerites of Waen-Fechan type. Occasionally, these 

 quartz- granules show idiomorphic contours towards chlorite 

 [C 240] ; in other examples the quartz of the intergrowth may be 

 in optical continuity with these larger granules. 



1 ' Geology of the Land's End District ' Mem. Geol. Surv. 1907, p. 35. 



2 E. H. Hatch., ' Textbook of Petrology ' 1909, p. 219. 



:i T. G. Bonney & E. Hill, Q. J. G. S. vol. xxxiv (1878) p. 217, and ibid. 

 vol. xlvii (1891) p. 84 ; also J. J. H. Teall, ' British Petrography ' 1888, 

 p. 270 ; and W. W. Watts, ' Charnwood Forest ' Geol. Assoc. Jubilee Vol. ii 

 (1910) p. 717. 



