A basic igneous sill occurring in the coal seams at Landywood, England, has been studied by means of cores from five boreholes. The sill was encountered at depths between 355 and 710 feet and varies from 117 to 181 feet in thickness. It is probably of the same upper Carboniferous age as many other igneous occurrences in the area. The sill represents a single intrusive period and consists primarily of microteschenite with a 50-foot teschenite zone in the upper part. Where the sill is thickest, crinanite and analcime-olivine dolerite zones developed from a drier part of the magma near the feeder pipe, which did not spread throughout the sill. Chlorite and carbonate zones developed along the outer portions of the sill because the magma had a high water, carbon dioxide, and alkali content. The chemistry of the development of these zones, and particularly the conditions favorable to the retention of the high water content within the magma are discussed. Two chemical analyses, one of ana