The. Petrography of the Intrusive Diabase. 89 



Iron-Ores. — These are present to a greater or less extent in all the 

 thin sections studied, and hence may be regarded as essential minerals. 

 As a rule, the ore present is titaniferous, and in the majority of cases it 

 is either ilmenite, or very closely allied to ilraenite in composition and 

 properties. In places, as for instance, near Malali on the Demerara 

 River, and near the Sir Walter Raleigh Mine at Arakaka, on the Barima, 

 the ore present is a titaniferous magnetite, and allowances have to be 

 then made by surveyors for the influence of that mineral on the indica- 

 tions of their compasses. These iron-ores frequently carry gold in small 

 quantities. 



Apatite. — Minute stout prisms and microlites of apatite are of not 

 unfrequent occurrence in the diabase, but are never present in more than 

 very small proportions. 



Biotite. — On the peripheries of some of the masses of pyroxene small 

 areas of a rather light brown mica may be seen, and these are more 

 especially noticeable in the vicinity of grains of iron-ore. This biotite 

 is not of secondary origin, but is a product of the original magma as it 

 became more acidic in composition by the crystallisation from it of the 

 iron-ore, labradorite, and pyroxene. 



Hornblende. — Brown basaltic hornblende is a mineral of very rare 

 occurrence in the diabase ; but perijiheral areas of green hornblende, 

 apparently original, are occasionally present on the pyroxene-masses. 

 I attribute their presence to the same cause as that of the biotite — the 

 gradual change in composition of the magma during the solidification 

 of the rock. 



Micro-pegmatite. — Areas of this aggregate are common in the 

 diabase, but only in a few specimens is it present in more than unim- 

 portant quantity. It consists of microscopical intergrowths of quartz 

 and feldspar, the latter being either orthoclase, microcline, or, more 

 frequently, anorthoclase. Where these intergi'owths are fairly plentiful, 

 the rock tends to pass through increase of quartz into quai'tz-diabase. 



Quartz. — Where present it is usually intergrowu with feldspar as 

 micro-pegmatite, but in specimens from a few localities the micro-pegma- 

 tite passes into irregular areas of original quartz. 



Jlicrodine. — -In some thin sections cut from specimens taken from 

 the more central parts of certain of the great dykes which are intersected 

 by the Demerara River, above Malali, the areas of micro-pegmatite pass 

 into small irregular patches of feldspar, showing very clearly twinning 

 of the microcline type. 



Olivine.^In a few specimens only has this mineral been noticed. 

 It forms clear glassy blebs or crystalline grains of high index of refrac- 

 tion, which, under crossed nicols, show strong polarisation-colours. In 



