. The geology of the goldfields of British Guiana. cross. The mass of the rock is coarse in texture, inplaces almost approaching a gabbro in structure, while its marginsai-e comj^act and fine-grained. It is intersected by later d3^kes andtongues of diabase and of basaltic diabase, or tholeite, running, as a rule,east and west. One of these, a coarse diabase, to the south of thecataracts, is about twenty-five feet in thickness ; another, near themiddle of them, is about a foot across and traverses both the diabaseand the (|uartz-p(jrphyry at the foot of the cataracts ; whilst between themis a t
. The geology of the goldfields of British Guiana. cross. The mass of the rock is coarse in texture, inplaces almost approaching a gabbro in structure, while its marginsai-e comj^act and fine-grained. It is intersected by later d3^kes andtongues of diabase and of basaltic diabase, or tholeite, running, as a rule,east and west. One of these, a coarse diabase, to the south of thecataracts, is about twenty-five feet in thickness ; another, near themiddle of them, is about a foot across and traverses both the diabaseand the (|uartz-p(jrphyry at the foot of the cataracts ; whilst between themis a third which consists of compact basaltic diabase, eight to ten inchesacross, having a columnar structure at right angles to its walls. Thesedykes evidently were injected into cracks in the main mass after itsconsolidation. The fine-grained diabase on the north-west of the headof the cataracts passes into a compact epidiorite, proljably l)y ccmtactaction with the acidic rocks throu2;h Yvlrich they were intruded. About [_To face page 15S. Fl\t-e UPPER PART OF TUMATUMARI CATARACTS (in KAINY SEAS0>), POT ARC RIVER. Pliofo bii C. If. Aiiderfoh Thfi J\)f(tr() ((ltd KiiiiihroiKj Rivers. 159 half a mile west of Turaatumari are small rapids caused l)y a dyke ofdiabase running parallel to the main one at the falls, whilst diabase isexjiosed at intervals for about one mile west of the rapids. About two miles west of the cataracts the river traverses a broadmass of fine-grained epidiorite, and exposures of similar rock occur atintervals for about six hundred yards, the general trend of the rocksbeing north-west and south-east. During the next two miles diabase isseen at intervals, apparently forming portions of dykes trending north-east and south-west. The low-lying banks between the mouth of theriver and about six miles west of Tumatumari consist largely of alluvialand fluviatile deposits. About five miles west of Tumatumari theriver crosses a belt of schistose quartz-porphyry, fir
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectgeology, booksubjectp
