and occurring in large ophitio plates. Both minerals pass over into green 

 alteration products. Plagioclase occurs in the form of prisms and irregular 

 plates of secondary consolidation. In the former condition it often penetrates 

 and is sometimes completely enclosed within the bisilicates. Iron ores are 

 very abundant. Olivine cannot be recognised, so that the rock has no place 

 in the true picrites. As it appears, however, to be intimately associated with 

 them it has seemed desirable to refer to it in this connection. If we regard 

 the felspar as accessory this rock must be classed with that division of Group 

 A, which is characterised by the absence of olivine. 



The facts collected by Mr. TANYXEY prove very conclusively that we 

 have in the greenstone ridge of Penarfynydd a mass of rock of varying 

 lithological character. Some varieties consist very largely of olivine and 

 approximate in character to the typical peridotites, others contain a smaller 

 amount of olivine, and others again are without that mineral. Brown 

 hornblende, augite and felspar, together with iron ores and apatite are the 

 other constituents, and these also vary in amount in different portions of the 

 mass. 



"We have now to consider those rocks for which the term hornblende- 

 picrite was first proposed. It must be remembered that they 

 differ from the original picrites of TscHKRMAK in containing a much 

 smaller amount of olivine, and consequently a lower percentage of 

 magnesia ; 15 or 16 as against 23 per cent. Certain varieties have been 

 described by Mr. TAWXEY (1 ' under the names otivine-diabase and /tontblende- 

 (1i<ib(tH<>, and by Mr. CLIFTON AVAiin' 2 ' as (tioritc. 



The term hornblende-picrite was introduced in 1881 by Professor BOXXEY, 

 under circumstances that have been already explained, and has since been 

 extended to several rocks occurring as boulders or /// xitu in Wales 

 and the Lake District. The most conspicuous macroscopic features of these 

 rocks are a black or greenish black colour, lustre-mottling, and the absence 

 or rarity of any definitely recognisable felspar. The constituents as seen 

 under the microscopic arc brown, green and colourless hornblende ; colourless 

 or nearly colourless augite ; olivine or, more frequently, pseudomorphs after 

 this mineral ; felspar or secondary products after felspar ; iron ores and 

 apatite. The brown hornblende usually occurs in large ophitic plates and 

 is the mineral which determines the lustre-mottling ; sometimes it shows 

 definite external form. The green variety sometimes appears to have been 

 formed by a mere change in the colour of the brown, and sometimes it occurs 

 in more or less independent needle-like (actiiiolitic) forms. The colourless 

 variety sometimes appears to have been formed by the bleaching of 

 the green variety, and sometimes unquestionably occurs as an independent 

 mineral of secondary origin. Augite occurs in grains, plates and well formed 

 crystals. Its characters have been already described. Biotite occurs 

 sparingly. Fresh olivine is very rare. It has been recognised only in the 



(1) G.M. Decade II., Vol. VII., p. 211 and Vol. IX., p. 458. 



(2) Geology of the Northern Port of the English Lake District. Mem. Geol. Survey. 1376. 

 p. 36, 



