CLASSIFICATION The following classification of the rock formations of British Guiana appears to be most appropriate at the present time and has been used on the British Guiana part of the map. Quaternary continental deposits (not shown on map) River alluvium Terraces, alluvial fans, and screes Residual deposits Quaternary marine deposits (Qm) Coastal fluviomarine alluvium Undivided marine deposits (T) Coastal sedimentary and White Sand series Laterite and bauxite deposits Post-Roraima formation intrusives (MZi) Younger basic intrusives: noritic gabbros, gabbros, and dolerites (diabases) Triassic (?) continental deposits (TRrc) Roraima formation Paleozoic (?) intrusives (PAi) Younger acid intrusives Late Precambrian (lpC) Volcanic series Early Precambrian (epC) Gneisses and schists DESCRIPTIONS OF FORMATIONS EARLY PRECAMBRIAN Gneisses and schists.—In the crystalline basement large areas consist of strongly foliated gneiss and schist. These are more in evidence at places remote from the Roraima formation, from which they may be separated by intervening extrusive and sedimentary rocks of the Volcanic series. They are of variable composition and include granitic gneiss, hornblende, and biotite gneiss, amphibolite, and pyroxene granulite. The latter occur in the Kanuku Mountains along with granitic gneisses with considerable garnet. In this area and farther south near Dadanawa the gneisses contain bands of a dark-gray to black garnetiferous pseudotachylite. The gneisses merge into finer-grained sericite, chlorite, talc, hornblende, staurolite, and kyanite schists and phyllites and into the foliated Volcanic series, and no clear delimitation of these has yet been practicable. Generally speaking the gneissose areas are poor in gold and other economic minerals,