Page:Popular Science Monthly Volume 79.djvu/428

424 steep to the northwest. The main topographic features of the region have a general northeast-southwest trend, and are related in a broad way to the structure. But detailed study of small areas within the field emphasizes very clearly that the structure is much more complex than is indicated by a general study of the field. This complexity is due to folding, to faulting, and in the northeastern part of the field, to associated igneous rocks. Apart from a few well-defined lines of faulting with northeasterly and southwesterly trend, there are many small faults running in various directions. Moreover, within small areas in the field the strikes and dips are often irregular. The rocks

are frequently very much broken and jointed, and slickensided surfaces, especially in the coal, are common.

This complex structure was produced by the intense crustal movements to which the rocks were subjected during late Tertiary or post Tertiary time. Recent subsidence of the region is indicated by the presence of alluvial deposits, in places several hundreds of feet in thickness, in the broad valleys now occupied by comparatively small streams.

The coal beds are restricted to the Kushtaka formation which has a known surface area of about 50 square miles. The evidence is fairly clear that an additional area of more than 20 square miles underlies the Tokun formation at varying depths. The coal beds are distributed throughout the thickness of the Kushtaka formation. "Where sections