92 The Geoloijij of the Gold Fields of British Gidana. 



abundance, whilst micro-pegmatite, and especially patches of quartz, 

 are of very rare occurrence. In such specimens enstatite is usually 

 present in greater quantity than it is in the more feldspathic parts ; 

 and in some, small granules of olivine or of its alteration-products are 

 sparsely distriliuted in the pyroxene masses. A few granules of sphene 

 are jn-esent in some specimens. 



The diabase of medium to fine-texture is made up of the same 

 minerals and aggregates of minerals as are the coarser-textured 

 varieties. Its range of specific gravity — from 2' 93 to 3 '01 — is not so 

 wide as is that of the latter. It is composed of labradorite in short 

 and narrow, lath-like crystals, with some, apparently of earlier con- 

 solidation, in short and l)road plates, in places showing pericline 

 twinning ; interstitial micro-pegmatite in varying but always small 

 quantity ; and many small ophitic aggregates of pale to almost colour- 

 less augite, with, in places, a very little peripheral green hornblende, 

 or with a few small plates of brown biotite. Granules of magnetite and 

 of titaniferous iron ore are irregularly distributed in small grains 

 through the rock, while apatite, in minute prisms, forms a rare 

 accessory. 



Olivina-didbase. — Typical olivine-diabase is of rare occurrence in 

 the parts of British Guiana which I have visited. The best examples 

 occur at the Stop Off Rapids in the Cuyuni River, at the Hiari Rapids 

 in the Aranamai Creek of the Pomeroon River, and in a dyke in 

 Groete Creek. It differs in appearance from the ordinaiy diabase of 

 the district by showing in hand-specimens small yellowish-green to 

 brownish-green blel)S with a greasy lustre, scattered through the mass 

 of the rock. The rock has a specific gravity of 3-00. Its structure 

 is similar to that of the diabase, with the exception that enstatite is 

 not present in the masses of pyroxene, that micro-pegmatite is seldom 

 present, and then only in minute quantity, and that the masses of 

 augite enclose many granules varying in size of almost colourless 

 olivine, which are traversed by frequent cracks, and are in places in 

 part, or entirely, changed to green serpentine. 



Quartz-diabase and Augite-yranophyre. — In places by a gradual 

 decrease in the proportion of the ferro-magnesian minerals and an 

 increase in the amount of feldspar and quartz and in that of micro- 

 pegmatite, the rock passes into quartz-diabase, or into an augite- 

 granophyre. Of the former the inner parts of a broad dyke of 

 diabase in the lower part of the Kuriebrong River are excellent 

 examples. The outer parts of this dyke are of normal structure, and 

 have a specific gravity of 2'93, whilst the inner portions have a specific 

 gravity of 2*77. The latter are of a coarse, almost granitic, texture, 

 are light-grey in colour, and show large crystals of labradorite, with 

 here and there small patches of pyroxene. In thin sections the rock 

 is seen to be made up of large plates and of lath-like crystals of an 

 andesine-labradorite, with an abundance of micro-pegmatite passing 

 in places into patches of quartz — some of which are granular — ^and a 

 relatively few ophitic aggregates of very pale, almost colourless, augite. 



