90 The Geology of the Gold Fields of British Guiana. 



hand-specimens the oHvine, where noticeable, is of a very pale greenish- 

 yellow or orange-yellow tinge, but in thin sections it appears colourless. 

 Tt is of rare occurrence in more than very unimportant quantity, and 

 T have found very few specimens of true olivine-diabase. In the Guiana 

 diabase the place of olivine is usually taken by light-coloured enstatite ; 

 but small blebs of green serpentine in some of them indicate the former 

 presence of olivine. 



Microlites. — These are present in some quantities in parts of the 

 diabase. They generally appear to consist of feldspar-needles, but in 

 some cases they may be needles of augite, or of light-coloured amphibole, 

 while some are probably apatite. 



The secondary minerals in the diabase are usually confined to the 

 edges of dykes, and to places where the rock shows signs of commencing 

 weathering. None of these minerals are at all common, with the 

 exception of hornblende, which, in some sections of the rock from the 

 edges of dykes, takes more or less completely the place of the augite. 

 It there occurs in the form of radiating fibres, or minute prisms 

 resembling spherulites. 



(a.) Mountain Masses. 



The diabase of the mountain masses varies in specific gravity from 

 2"93 to 3'17 according to the relative proportions of the feldspar and 

 the pyroxenes ; the masses are coarse-textured rocks which, as a rule 

 in hand-specimens, appear to be made up of blebs and porphyritic 

 masses of pyroxene in a ground mass of white feldspar. Some 

 specimens in which the pyroxene and feldspar are present in approxi- 

 mately equal proportions have a somewhat granitic appearance. 



Under the microscope they are, as a rule, more or less granitic in 

 structure, but the pyroxenes always are of later consolidation than are 

 the feldspar, laths of the latter being in places included in the ophitic 

 masses of pyroxene. The augite is generally present in two varieties, 

 of which the pale brown one is almost always by far the more plentiful ; 

 diallagic markings are not uncommon in the pyroxenes, whilst enstatite 

 in places, with faintly marked dichroism of the hypersthene type, is 

 frequently present in some quantity. Many specimens contain more 

 or less interstitial mici'O-pegmatite with, in places, small areas of original 

 quartz. The pyroxene-masses are frequently accompanied by small 

 (juantities of peripheral green hornblende and by flakes of brown 

 biotite, and as this is of more frequent occurr-ence in the rocks in which 

 micro-pegmatite is relatively more abundant than in those in which 

 it is rare or entirely absent, I consider that, as already stated, those 

 minerals are jorobably original, and that their presence may be due to 

 the matrix having become of a more acidic nature after the separation 

 of the bulk of the ferro-magnesian minerals. In places, however, the 

 biotite is clearly due to a resorption of some of the magnetite — a mineral 

 of first separation. A separation from the pyroxenes of hornblende 

 in fibrous aggregates is also noticeable in a few specimens, and the 

 mineral is then clearly of secondary origin. 



