OE LITTLE KNOTT (cFMBEELAND). 51 9 



the neighbourhood of St. Davids, and to him I am indebted for speci- 

 mens and for the following note as to the locality. He says : — " The 

 boulder is somewhat rounded ; its longer axis, which lies nearly S.E. 

 and jN".W., measures about a yard. A transverse section is slightly 

 triangular, the shorter sides measuring respectively about 16 in. and 

 22 in. It lies on the promontory forming the east side of Porthlisky 

 harbour *, resting immediately on Dimetian rock, surrounded by an 

 uncultivated area overgrown by gorse and heather. The striae 

 along this coast usually point from IST.^V.to S.E. ; but it is clear that 

 very many of the boulders scattered over it must have come from 

 the high land in the jNT.E. of Pembrokeshire, the Precelly range. 

 There is ample evidence of local till, and in places (at considerable 

 elevations) of marine sand with transported boulders, fragments of 

 flint being common among them. I fancy this points to the deriva- 

 tion of some of the materials, including possibly certain boulders, 

 from a JS'.W. source." 



The surface of the boulder (which is overgrown with lichens) is 

 rough and lumpy. A brown staining extends inward for less than 

 •1 in. The rest of the rock is in good preservation and is wonder- 

 fully like that of the Pen-y-Carnisiog boulder, looking perhaps even 

 fresher and thus closer to the Schriesheim rock. The larger cr3'Stals 

 of hornblende are sometimes quite | inch in diameter. As in 

 the other cases the general colour is a dull somewhat mottled green, 

 with a few whitish specks f. The hornblende crystals on a smooth- 

 cut surface have a slightly metallic and silvery lustre. My previous 

 papers render a minute description of the microscopic structure 

 unnecessary, so, referring generally to them and especially to the 

 account of the Pen-y-Carnisiog specimen, I will only call attention 

 to one or two points of difference. There are a few grains of olivine 

 fairly well preserved (Plate XYI. fig. 4). The larger hornblende crys- 

 tals, in addition to the serpentinous enclosures, which in all proba- 

 bility have replaced olivine, contain a fair number of crystalline grains 

 of a colourless augite. This mineral also occurs in considerable 

 quantities in the slide, frequently with a tendency to grouping. 

 There the crystals are sometimes set in a serpentinous ground-mass, 

 and sometimes associated with a pale-coloured hornblende, and 

 occasionally with grains of another variety of a serpentinous mineral 

 which appears to be subsequent in consolidation. This augite has 

 been already noticed in my former papers ; but in the cases there 

 described it was often not very definite in form or cleavage, and had 

 a dusty look as if partially decomposed. Instances of the latter 

 variety occur in these slides also ; but the majority of the crystals 

 are beautifully clear, and are well defined, showing characteristic 

 transverse and longitudinal sections and cleavage, and giving such 

 clear brilliant colours with crossing nicols that at the first glance 

 one might take them to be olivine. They appear to have a slightly 

 granular, fibrous or silky structure (Plate XVI. fig. 6). They must, 



* Immediately south-west of the letter S in the Survey map. 

 t These appear to me to be too soft for felspar ; probably they are a steatitic 

 mineral, like that which mottles certain serpentines. 



