88 The Geology of the Gold Fields of British Guiana. 



in all varieties is made up of pyroxene and of plagioclase-f eld spar, iron- 

 ore being present only in very subordinate quantity. 



The accessory minerals I have found in the diabase of the colony 

 are as follows : — 



(a) Original. — Enstatite, biotite, hornblende, micro-pegmatite, 

 quartz, microcline, magnetite, ilmenite, apatite, olivine, glass 

 and microlites. 



(6) Secondary. — Epidote, hornblende, biotite, chlorite, (juartz, 

 zoisite, sericite, leucoxene, sphene, serpentine, limonite, pyrite, 

 zeolite and carbonates. 



Pyroxene. — In thin sections two pyroxenes can be distinguished : — 

 augite, the essential constituent of the rock, and enstatite, which is 

 frequently present as an accessory, but seldom or never in important 

 quantity. The augite is usually a pale, almost colourless variety, but 

 in places it has a violet tinge. In some sections the usual augite is 

 accompanied by another, also a colourless one, which has a inuch higher 

 double refraction. It is only present in any quantity in specimens 

 obtained from the mountain-masses or from the more central deeper- 

 seated parts of the larger dykes. In these it tends to be moi-e idio- 

 morphic than does the common kind ; it has a good prismatic cleavage, 

 and in places shows well-marked diallagic stri;e. In some sections 

 the two j^yroxenes are seen in micro-pegmatitic relationship with each 

 other. It yields more easily to weathering agencies than does the 

 common kind ; and in some specimens in which the latter is quite fresh 

 and unaltered, the former shows signs of alteration into bastite and 

 into serpentinous products. 



In thick sections the common augite of the diabase has a pale brown 

 colour, and is of the aluminous type usually found in diabase. It 

 always occurs in more or less allotriomorphic irregular masses enclosing 

 lath-shaped sections of plagioclase. This shows that it was of later 

 consolidation than the feldspar. 



In the coarser-textured rocks enstatite, showing to some extent the 

 pleochroism of hypersthene, is present as an accessory. Ordinary 

 enstatite, distinguished from the augites by its low double refraction 

 and straight extinction, is seen in small quantity in sections cut from 

 the diabase of many of the dykes ; and from its mode of occurrence it 

 has been generally of earlier consolidation than has the augite. 



Lahradorite. — The plagioclastic feldspars present in places are of 

 two periods of consolidation, the earlier being somewhat more basic than 

 the later, but both may fairly be described as " labradorite," or in some 

 cases as "bytownite." As a rule, the feldspars are present in the form 

 of prisms giving, in the thin sections, lath-shaped areas. They are 

 commonly twinned after the albite type. The lath-shaped sections, 

 generally, do not show any zonal structure, but this is well-marked in 

 the porphyritic crystals in the somewhat rare rocks of the porphyritic 

 diabase type. 



