516 PEOF. T. G. £OjS"XET ON THE SO-CALLED DIOEITE 



below 5) is from a portion of or,e of the most southerly dykes, which, 

 in the rest of the mass, is of the usual dark green blotched colour («). 

 There is no sharp line between the two, but the light rock shades 

 rapidly into the other as if it represented a difference in the 

 material 'boiled up/ a defect in the mixing, a local accident 

 affecting the cooling and crystallization or subsequent superinduced 

 alteration, but not a later-intruded dyke. The Pengorphwysfa boss 

 is a similar but larger mass, and shows the same but less marked and 

 rapid variations. The boulders from Caemawr are found trailed to 

 the south for miles. I was not looking out for them, but my 

 impression is that they came to hand right up to Pen-y-Carnisiog 

 and beyoDd."— T.M'KH. 



The specimens from the Pengorphwysfa boss vary from an ordinary 

 compact " greenstone " to a moderately coarse rock consisting of 

 hornblende crystals from -2 to -3 inch wide, and a greenish grey 

 mineral. Under the microscope, hornblende is seen to be very 

 abundant. There are, as above, brown crystals passing into green 

 and containing serpentinous enclosures, with numerous smaller 

 crystals and fibrous flakes thickly scattered over the slide and 

 giving brilliant colours with polarized light : interspersed with the 

 smaller of these is some felspar iu small grains of irregular form 

 and indefinite character, not indicating an ophitic structure, and 

 decidedly less abundant than in the Little-Knott rock. Certain 

 of the serpentinous grains with reticulate minute belonites suggest 

 the former presence of enstatite rather than of olivine *. There are 

 some grains of iron peroxide ( ? haematite), and perhaps a little 

 apatite. 



A second slide, cut from a slightlj'' more mottled specimen, 

 contains a rather larger quantity of a decomposed idagioclastic 

 felspar. 



One variety (a) of the rock from the southern dyke at Caemawr 

 is macroscopically much like the last described, but on microscopic 

 examination it presents this difference, that very little felspar can 

 be detected ; there is a fair amount of a pale yellowish serpentinous 

 mineral, or rather of aggregated groups of small serpentinous mine- 

 rals, which have most probably replaced olivine, or, perhaps in one or 

 two cases, enstatite. Another peculiarity is that there are a fair 

 number of crystalline grains of a mineral with its external angles 

 not very well defined, often occurring in small groups. It is clear 

 and almost colourless, though its cleavage and other cracks are often 

 much stained, apparently by the formation of a brown decomposition- 

 product (Plate XYI. fig. 1). It is one of the pale-coloured augites t. 



Another specimen (b) from this dyke exhibits a variation as 

 marked as any described in the Little-Knott rock, being a mottled 

 dull green and -whitish rock, a fairly typical specimen of the horn- 

 blendic gabbros frequent in jSTorth Wales. Under the microscope, 

 felspar is seen to predominate, the crystals varying in size, so that 

 the structure might be called porphyritic. Most of it is plagioclase, 



* This is well seen in a slide lent to me by Prof. Hughes. 



t A slide lent to me by Prof. Hughes confirms the above description. 



