brown in the former position, and perfectly colourless in the latter. These 

 give, under crossed nicols, a vivid chromatic polarisation. The striking 

 contrast between the two varieties is that one is brown when viewed with 

 light vibrating parallel to the cleavage cracks, the other shows a similar but 

 deeper colour when viewed with light vibrating at right angles to these 

 cracks. 



The pleochroism of chlorite is thus defined by RoeENBUSCH ; (rays 

 vibrating at right angles to the vertical axis), green. E (rays vibrating 

 parallel with the vertical axis), yellow, red or brown. The pleochroism 

 of the green variety above referred to agrees with this, and we 

 therefore regard it as a chlorite. The pleochroism of the nearly colourless 

 variety agrees with that of a bleached biotite. Iron oxides are plentifully 

 scattered through the micaceous and chloritic aggregates, and are sometimes 

 seen to be arranged parallel to the cleavage planes, as in the scyelite (see 

 fig. 2, Plate Y.) There is 110 original felspar, and it is very doubtful 

 whether any traces of that mineral exist in the form of pseudomorphs. 



The above specimen was collected by Professor SEDGWICK, and simply 

 labelled " below Penarfynydd." Mr. TAWXEY (l1 endeavoured to find the 

 rock in- situ, but was not altogether successful. The Geological Survey map 

 shows Penarfynydd as the seaward termination of a long mass of greenstone 

 extending north and south, for a distance of four miles. This mass was 

 carefully investigated by Mr. TAWXEY, who detected in it two varieties ; a 

 greenish black rock, very rich in olivine and poor in felspar, and a brownish 

 grey rock spotted with white, and containing a considerable amount of felspar. 

 The former (divine-diabase, TAWXEY) appeared to be intrusive in the latter 

 (diabase, TAAVXEY) and both were probably intrusive in rocks with Lower 

 Arenig fossils. 



Professor BOXNEY has since pointed out (2) that in all probability 

 Professor BEDOWICK'S specimen is merely a variety of TAWXEY'S " oliviiie- 

 diabase," and has referred the latter rock to his group of hornblende-picrites. 

 This rock shows marked lustre-mottling. It consists largely of olivine in 

 rounded grains which occur as enclosures in augite, hornblende and felspar. 

 Augite seems here to be somewhat more abundant than hornblende, so that 

 the rock approaches the augite-picrites. It occurs in large sheets and smaller 

 detached portions. Hornblende is present, and presents exactly the same 

 features as that of the rock from Penarfynydd, which has already been 

 described. Biotite is sparingly present. The felspar is mostly represented by 

 zeolitic and other pseudomorphs, as in the Inchcolm rock. 



Another variety of rock from the Penarfynydd ridge has been described 

 by Mr. TAWNEY under the name of hornblendic diabase (see Fig. 2, Plate 

 Till.) It occurs near Careg Llefain. In the hand specimen it is blackish- 

 brown to black, with cleavage surfaces of the bisilicates sometimes half an 

 inch across. Basaltic hornblende and a colourless or nearly colourless 

 pyroxene are the chief constituents, the former being the most abundant 



(1) Woodwardian Laboratory Notes, G.M. Decade II., Vol. VII.. p. 208. 



(2) Q.J.G.S., vol. XLL, p. 517. 



