220 Grenville A. J. Cole — Igneous Rocks of Stunner. 



terminates Stanner Kock that the dark mass is traversed by pale 

 intrusive veins of a very different nature ; and an examination of 

 the " greenstone " itself reveals a number of dykes and veins of 

 every variety of grain, reminding one of those cutting through the 

 gabbros in the mountains round Coruisk. No true lavas or scoria- 

 ceous products remain about the flanks of Stanner, as about Corndon 

 in the north or the Carneddau farther west ; we have to deal largely 

 with crystalline materials, which divide themselves at the outset into 

 a more acid and a more basic series. The former, clearly intrusive 

 in the latter, marks the later stages of eruption. 



With the relations of these masses to the surrounding deposits I 

 am not as yet prepared to deal. Murchison's prediction that igneous 

 intrusions underlie the altered Woolhope of Nash Scar l has not, so 

 far as I am aware, been verified, and evidence of the penetration of 

 this crystalline limestone by igneous veins is wanting at Brook 

 Quarry and Yat Hill. I have failed to find satisfactory junctions 

 between Stanner and Hanter Hill; but may add that I have not 

 found pebbles from these hills in the ancient conglomerate of Old 

 Eadnor, which underlies the Woolhope Limestone. It is yet un- 

 certain whether the local alteration can be attributed to faulting and 

 to crushing down against hard earlier igneous masses, or whether it 

 is due, as Murchison held, to the intrusion of those masses in times 

 later than Silurian. 



I. The Acid Semes. 



The most highly crystalline member of this series is a pale pink- 

 grey pegmatite — the word being employed in its original sense 2 — 

 which passes from a well-developed graphic granite through finer 

 varieties to a practically micropegmatitic form. This gradation is 

 beautifully seen on the edge of a vein in Stanner cliff, of which 

 a microscopic section has been prepared ; where the cooling has 

 been fairly rapid, along the line of junction with the more basic 

 rock, a selvage of minuter graphic structure occurs, graduating off 

 in a distance of 3 millimetres into the normal coarser form (Fig. 1). 



This graphic granite is best developed in veins, not many inches 

 wide, on the southernmost summit of Stanner Eock. Under polar- 

 ized light the felspar presents the microcline structure, and extin- 

 guishes — as one would have inferred from its cleavage-surfaces on 

 the rock-mass — uniformly over considerable areas. The areas of 

 entangled quartz show similar optical connexion, as is the case in so 

 many pegmatites, as though we dealt, not with isolated inclusions 

 in a felspar, but with a complex intergrowth of large quartz and 

 felspar crystals. Some portions exhibit a lamellar arrangement of 

 the two constituents; but this appears to have no relation to 

 proximity to the edges of the vein. 



The rock is much cracked and faulted, minute bands of fault- 

 rock appearing in microscopic sections. Hornblende prisms, and 

 perhaps tourmaline, occur in a subordinate manner. 



The less crystalline members of the Acid series are found in 



1 See " Siluria," 5th edition, p. 108. 



2 Hauy, Traite de Min. 2nde edit, tome iv. p. 536. " De irnyfia, c'est-a-dire 

 renfermant des pieces qui sont comme fiche'es dans une matiere principale." 



