81 



In 1880 Dr. A. GEIKIE (1) recognised the existence of rocks of this class in 

 two localities near Edinburgh; at Blackburn, near Bathgate and in the island 

 of Inchcolm. In both cases the rocks are associated with strata of 

 carboniferous age. Felspar often occurs as an accessory constituent of the 

 picrites, and by the gradual increase in the amount of this mineral the rocks 

 of this class shade off into the olivine dolerites. 



We have now to consider a further extension in the meaning of the 

 term picrite. In 1881 Professor BoxNEY, (2) accepting the " Schillerfels " 

 of Schreisheim in the Odenwald as a typical picrite on the authority of 

 Professor ROSENBUSCH, called attention to a rock of similar character occurring 

 as a boulder near Pen-y-Carnisiog in Anglesea. A microscopic examination 

 proved that the dominant constituent of this rock was hornblende and not 

 diallage, the mineral which was then regarded, on the authority of Professor 

 COHEN, as the principal constituent of the Schreisheim rock. The most 

 conspicuous macroscopic feature of the Pen-y-Carnisiog and Schreisheim 

 rocks is the great abundance of conspicuous cleavage surfaces, often 

 measuring two-thirds of an inch in length, of a black mineral. The reflection 

 from these surfaces is, however, interrupted at numerous points by the 

 presence of enclosures ; a feature which is common to many basic and ultra- 

 basic rocks, and which was specially mentioned by STREXG l3) in 1862 as a 

 characteristic of the Bastite rock of the Harz. Professor PUMPELLY (4) called 

 attention in 1878 to the same feature in the basic igneous rocks of the Lake 

 Superior region, and proposed for it the term lustre-mottling. This term was 

 adopted by Mr. IRVING (5) in his detailed description of the same rocks. Mr. 

 G. H. WILLIAMS has recently proposed the term poicilitic for the same 

 feature ; but as PUMFELLY'S term has priority and is moreover intelligible 

 to everybody, there seems no sufficient reason for introducing a new term to 

 the already over-burdened nomenclature of petrographical science. 

 Lustre-mottling, then, is due to the enclosure of certain minerals in large 

 irregular crystalline individuals belonging to one of the bisilicates ; 

 augite, enstatite or hornblende. In Scyelite it is due to corres- 

 ponding enclosures in a uiiiaxial biotite. In basic rocks (e.g., dolerite 

 of Carreg-y-rimbill, Pwlhelli) the enclosed mineral is usually felspar; 

 in the ultra-basic rocks (e.y., Pen-y-Carnisiog rock) it is either olivine or its 

 serpeiitiiious pseudomorphs. 



To return to the Pen-y-Carnisiog rock. Seeing that the dominant 

 bisilicate was hornblende and that in this respect it differed from the true 

 picrites, Professor BOXXEY proposed the name of hornblende-picrite. In 

 the same paper Professor BONNEY refers to a remarkable rock collected by 

 Professor SEDGWICK near Penarfynydd in the Lleyn peninsula, Caernarvon- 

 shire, as belonging to the same type. The slide in the Woodwardian 



(1) Trans. Roy. Soc., Edin., Vol. XXIX. (1880), pp. 504-507. 



(2) Q.J.G.S., Vol. XXXVII., p. 137. 



(3) N. J., 1862, p. 533. 



(4) Proc. Amer. Acad., Vol. XIII. (1878), p. 260. 



(5) The copper-bearing rocks of Lake Superior. Monog., U.S. Geol. Survey, Vol. V., 

 1883, 



