The Petro(jrn))hy of the Fmidampnial Ow.issose Complex. 47 



Talc-schist. — In the Cuyuni River, south-west of Swarima Island 

 near Wariri, hornbleude-granitite-gneiss is traversed by a broad dyke 

 of zoisite-amphibolite, the inner part of which, for about 500 yards 

 in breadth, consists of a mass of light-coloured hornblende-rock, 

 parts of which are more or less altered, being in places changed into a 

 pale-greenish rock, having a soapy feel and being easily scratched by the 

 fingei'-nail. 



The amphibolite is a greenish-grey rock and varies from fairly 

 compact to porphyritic. The latter is made up of relatively large 

 crystals of a colourless amphibole in a confused grey matrix of zoisite 

 and feldspar, the strire of plagioclase being here and there faintly 

 visible. The amphibole-phenocrysts usually have frayed-out edges, and 

 in places a little chlorite is developed in them. The specific gravity of 

 the rock varies from 2-86 to 2-97 according to the relative abundance 

 of hornblende it contains. The amphibolite where not altered has a 

 specific gravity of 3-01, and is made up of a very pale-blue, almost 

 colourless, hornblende with some smaller crystals of a green one. The 

 edges of the larger masses are actinolitic, and the masses contain in 

 places prisms of epidote. Between the masses of hornblende are some 

 irregular areas of epidote and zoisite. A few minute crystals of 

 magnetite have been extruded from some of the masses of liornblende. 



Where the rock appears to be only very slightly altered its specific 

 gravity is 3-02, and it is made up of large masses of colourless horn- 

 blende somewhat altered, and having their cleavages marked by 

 abundant minute extruded grains of magnetite. Many of the masses 

 show a uralitic structure, and some contain patches of a very pale 

 chlorite. A little colourless pyroxene is present in places. 



Where the alteration commences to be readily noticeable the rock 

 has a specific gravity of 2-99, and is generally similar in composition 

 and structure to the very slightly altered rock, but grains of extruded 

 magnetite are more abundant, and areas of pale chlorite and of 

 serpentine are noticeable. Where patches of feldspar occur they are 

 invaded by colourless needles of hornblende and are rendered cloudy 

 by zoisite. Many flakes of talc and of sericite are found in places. 



The completely altered rock has a specific gravity of 2-9 1, the large 

 plates of hornblende being entirely metamorphosed, with extrusion of 

 magnetite, into a complex of talc, with some serpentine and chlorite. 



This is an interesting case of the alteration of a norite, or of a 

 gabbro, the pyroxenes of which were made up largely of the enstatite- 

 molecule, into a zoisite-amphibolite, and of the more basic parts into a 

 talc-serpentine rock or talc-schist. 



