412 C. A. Raisin — The Serpentines of the Lleyn. 



recognized, which are often shiny and palagonitic at their exterior. 

 In small quarries and knolls on the western face of the hill, the 

 rock is a green and purplish breccia. Although it presents some 

 resemblance to an agglomerate, I am doubtful whether this is the 

 true explanation. Many of the purple fragments are parts of 

 spheroids, or are spheroids which are fissured, with the fine greenish 

 matrix filling the interspaces. The microscope slides and the hand 

 specimens prove a fluidal structure, certainly within, and probably 

 outside, the small fragments, which even include some ordinary 

 brown tachylite. I believe that a second movement of lava took 

 place breaking a mass partially consolidated. This might have 

 occurred within the pipe or neck of a small vent, but it is probable 

 that the mass represents, at least in part, lava which flowed at the 

 surface. At the top of the hill the rock is less disturbed, and con- 

 sists, as shown by the microscope, of lath-shaped felspars within 

 a matrix containing very minute augite, magnetite, and other in- 

 cipient crystals. Thus we find at Careg gradations from tachylite 

 to a micro-crystalline condition. 



At one place on the eastern slope above the house is a green 

 diabase with green porphyritic felspars. Ilmenite crystals glisten 

 on the surface, and the slide exhibits the characters of a fine-grained 

 dolerite, with decomposed felspars, very fresh white augite, and a 

 constituent now green and serpentinised. The rock is doubtless 

 intrusive and may be of later age. 



8. West of Hendre-uchof. — The patch of rock shown on the word 

 Cil y bwth was probably exposed in a quarry, now completely 

 grassed and marked only by a curve in the sloping ground ; and the 

 outcrop near Bachwen I also failed to see. 



West of Hendre-uchaf I obtained specimens of an old basalt or 

 diabase, compact in structure, with patches of purple and greenish 

 colour. Microscopic examination shows the darker part to consist 

 of lath-shaped felspars amid a black deposit of magnetite. The 

 felspars exhibit a somewhat spherulitic grouping at places especially 

 within the ferruginous aggregates. Small porphyritic crystals, often 

 fractured or incomplete, slightly dichroic, yellow in colour, now 

 serpentinised, are probably a hydrous ferro-magnesian silicate. A 

 similar mineral forms small acicnlar or shuttle-shaped microliths.^ 



The rocks in the two remaining examples are clearly clastic. 



9. North of BraicJi ancheog. — (Annelog). This small patch is a 

 purple and green agglomerate. The rocks included, as seen on 

 examination with the microscope, are basalt or andesite, consisting 

 chiefly of felspar microliths, with more or less viridite or magnetite. 

 Pyrite and secondary limonite occur. The pieces are rounded or 

 sub-rounded, separated by palagonite; and calcite is deposited both 

 within and between them. In the small quarry, the fragments are 

 of no very great size, the larger being about six or seven inches 



1 See figures in Brit. Petrog. J. J. H. Teall, p. 14 (after Zirkel) ; U. S. 

 Explorations of 40tli parallel. Microo. Petrog. F. Zirkel, pi. i. fig. '20 ; Typical 

 Forms of Crystallites. F. Eutley. Miu. Mag. Dec. 1891, p. 268, fig. 17. 

 Crenulites. 



