282 PROF. A. H. COX A>~D ME. A. K. WELLS OX THE [vol. lxxvi, 



Many of the coarser-textured examples are beautifully ophitic 

 in their central portions, notably the Cae-Einion sill at the point 

 where it crosses the Gwynant, also the sill alongside the main 

 road west of Arthog. In other cases, or even in an intrusion that 

 is normally ophitic, it is not uncommon to find the augite tending 

 to form euhedral crystals. This takes place even in rocks so 

 coarse as to be gabbroid, as, for example, in parts of the Gelli-llwyd 

 intrusion ; such rapid changes in texture show how eventy balanced 

 were the conditions which determined the idiomorphism or other- 

 wise of the augite. 



The pyroxene is alwaj^s a pale variety, sometimes green, 

 sometimes pinkish brown [C 248]. l The felspar occurs in the 

 usual idiomorphic tabular crystals, appearing' as large plates or 

 laths according to the direction of section. It is near albite in 

 composition, and is usually altered to a greater or less extent, 

 being crowded with finely-divided epidote and zoisite. At several 

 localities hand-specimens of the rocks show numerous pale patches 

 measuring up to a centimetre or more in diameter. These patches 

 are almost opaque, except in thinnest sections when they are 

 resolved into an equi-granular mosaic of epidote and zoisite 

 replacing large felspars. Examples of this type occur locally 

 on Gelli-llwyd [C 247 & 250]; also at Bron-y-Gader, 2 where they 

 have been quarried for building-material. 



Contact-altered diabases. — The Gelli-llwyd diabase has been cut by 

 the later Crogenen granophyre, and has consequently suffered a certain 

 amount of contact-alteration. This has led to a partial clarification of the 

 felspars and to a complete recrystallization of the dark mineral which, 

 while originally augite, is now represented by pale hornblende crowded 

 with magnetite - grains. The hornblende has penetrated as long fibrous 

 crystals right through the larger felspars, and this recrystallization has 

 obscured the original texture, which was probably subophitic. The changes 

 seem to be so similar to those brought about in other cases of contact- 

 metamorphism of dolerites by granitic rocks that there is no need for 

 detailed descriptions. One curious feature of these contact-altered dolerites, 

 now in the form of albite -hornblende rocks, is their mineralogical similarity 

 to the marginal basic modifications of the granophyre with which they are 

 in contact. The two rocks are only distinguished by the altered dolerite 

 being of coarser grain (see p. 288 for details as to junctions). 



The spilite-like diabases. — A set of basic intrusions occurs in the 

 Lower Basic Series, being especially abundant among the lowest beds of the 

 series, and also appearing in beds below, that is in the Cefn-Hir Ashes and 

 occasionally in the Crogenen Slates. Individual intrusions are usually small, 

 with the result that the rocks are usually fine-grained and vesicular. In such 

 cases it is not easy to distinguish them from the pillow-lavas among which 

 they occur. As the intrusions increase in size, they pass over into rocks more 

 comparable with the normal diabases described above. It seems probable 

 that these intrusions are almost contemporaneous with the lavas, and that 

 they simply represent small masses of lava-material, which did not succeed 

 in finding their way to the surface, but consolidated under a shallow cover, so 



1 The numbers in square brackets refer to slides in our ' Cader-Idris ' 

 ■collections. 



2 G-. A. J. Cole & A. V. Jennings, Q. J. G. S. vol. xlv (1889) p. 432. 



