136 Reports and Proceedings — Mineralogicnl Society. 



they are mixed in various proportions. They do not represent a 

 composite intrusion, but simultaneous intrusions of an imperfectly- 

 mixed magma. There is no evidence of differentiation in situ, but 

 the facts suggest a common origin from a differentiated magma-basin. 

 The aplite- veins may represent the most acid phase of the differentiated 

 magma. 



Petrographically the rocks are of considerable interest, as exhibiting 

 types not very commonly occurring in the British Islands. They also 

 afford unusual facilities for the study of both rhombic and-monoclinic 

 pyroxenes, and appear to throw light upon the origin of the sahlite- 

 striation of the latter. Rhombic pyroxene generally crystallized 

 earlier than the monoclinic pyroxene, but sometimes these relations 

 are reversed, and often the two forms are crystallographically 

 intergrown, sometimes as twitis. There are two distinct varieties of 

 augite, distinguished by the presence or absence of a basal striation. 

 The relation of these two types lends support to the perthitic theory, 

 that there is an ultra microscopic crystallographic intergrowth of 

 rhombic and monoclinic pyroxene. The probable age of the intrusions 

 is not greater than that of the earth movements which folded the 

 Arenig Shales in this district. The observations recorded in the paper 

 seem to point to the conclusion that acid streaks and cores in basic 

 igneous rocks may not always be due to differentiation in situ. 



II. MiNERALOGICAL SoCIHTY. 



November 12th, 1907. — Professor H. A. Miers, F.R.S., President, in 



the Chair, 



On hopeite and other zinc phosphates and associated minerals 

 from Broken Hill Mines, North-Western Rhodesia ; b)- L. J. Spencer 

 (Geol. Mag., 1907, p. 379). Hopeite is abundant as brilliant water- 

 clear crystals or as larger white crystals reaching 2 cm. across. The 

 crystals are orthorhombic with a : b : c = 0'5786 : 1 : 0*4758. 

 Cleavage flakes parallel to the brachypinacoid show a zonal intergrowth 

 of two substances, di.stinguished as «-hopeite and /3-hopeite : these 

 differ considerably in their optical characters, and slightly in sp. gr. 

 (3 — 3"1) and the temperature at which water is expelled. Associated 

 with the hopeite crystals on the bone-breccia are brown botryoidal 

 masses of vanadinite. The other zinc phosphates occur, not in the 

 bone cave, but with cellular limonite and crystals of descloizite and 

 pyromorphite in connection with the zinc-lead ores (which consist of 

 an intimate mixture of cerussite and hemimorphite with interspersed 

 limonite). The new species tarbuttite occurs in great abundance, and 

 is a basic zinc phosphate, Zn^^ P„ Og . Zn (OH)o, with sp.gr. 4'15 ; 

 the crystals are anorthic with ac = 55° 50', ab = 84° 34', be = 76° 31', 

 c being a direction of perfect cleavage. Pseudomorphs of tarbuttite 

 after calamine (Zn C Og), descloizite, and hemimorphite ai'e not 

 uncommon. Another new species, named para]ioj)pite, has the same 

 chemical composition as hopeite, Zng Po Og . 4H2O, but is anorthic 

 with sp. gr. 3-31. The platy crystals somewhat resemble hemi- 

 morphite in appearance ; they have one perfect cleavage, approximately 



