The Bartica assemblage in British Guiana consists of a varied suite of gneisses and amphibolites on the northern margin of the Guiana shield, occupying more than 5,000 square miles. The rock groups in the assemblage are well exposed in the region of Bartica, and exposures in Kereti quarry, 12 miles north of Bartica, are described in this report. The results are held to be valid throughout the assemblage. It was found that the foliation and banding are of tectonic origin, and that the distribution of the rock types took place in anenvironment that was chemically mobile within the amphibolite facies. A two-phase metasomatism with successive additions of Na and K followed the metamorphism, and was in turn followed by intrusion of granitic dikes.