. The geology of the goldfields of British Guiana. ntly marked dichroism of the hypersthene type, isfrequently present in some quantity. Many specimens contain moreor less interstitial miciO-pegmatite with, in places, small areas of originalquartz. The pyroxene-masses are frequently accompanied by small(juantities of peripheral green hornblende and by flakes of brownbiotite, and as this is of more frequent occurr-ence in the rocks in whichmicro-pegmatite is relatively more abundant than in those in whichit is rare or entirely absent, I consider that, as already stated, thoseminerals are joroba
. The geology of the goldfields of British Guiana. ntly marked dichroism of the hypersthene type, isfrequently present in some quantity. Many specimens contain moreor less interstitial miciO-pegmatite with, in places, small areas of originalquartz. The pyroxene-masses are frequently accompanied by small(juantities of peripheral green hornblende and by flakes of brownbiotite, and as this is of more frequent occurr-ence in the rocks in whichmicro-pegmatite is relatively more abundant than in those in whichit is rare or entirely absent, I consider that, as already stated, thoseminerals are jorobably original, and that their presence may be due tothe matrix having become of a more acidic nature after the separationof the bulk of the ferro-magnesian minerals. In places, however, thebiotite is clearly due to a resorption of some of the magnetite—a mineralof first separation. A separation from the pyroxenes of hornblendein fibrous aggregates is also noticeable in a few specimens, and themineral is then clearly of secondary origin. X.— FINE-GRAINED DIABASE, x 40,TINAMU, CUYUNl RIVER. Pho/o by J. IfilHiims.
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