Dark to black limestone crops out at a number of places in the southern half of the Peninsula. Farther north, bluish-gray crystalline dolomitic limestone found in outcrops on the island of Sichang near Siracha and in low hills just northwest of Kančhanaburi is believed to be of the same age as the limestone of the Peninsula. Small disseminated crystals of pyrite are common in this limestone but, in contrast, very little or no pyrite is generally observed in the much younger limestone of Permian age. Nearly 9,000 feet of evenlybedded limestone have been measured at Thungsong. Clay and sand impurities mark the bedding planes, and thin discontinuous chert layers east of Thungsong contain markings resembling sponge spicules. West of Phatthalung sandy dark limestone weathering to sand has been found.
Kančhanaburi Series (Silurian, Devonian, and Early Carboniferous)
Clastic sediments, in many places metamorphosed to slates, phylite, and quartzite, extend from the Shan States border southward along the western side of Thailand to the Malayan border. In Northern Thailand the series appears to be as thick as in the south but tight folding and faulting make accurate measurement impossible. The rocks of the Kančhanaburi series are generally deeply weathered, but along stream beds the slate or shale, phyllite or argillite, and quartzite or sandstone are uniformly dark gray-green. The meager paleontologic evidence now available indicates that the Kančhanaburi series contains rocks that range in age from probable Silurian to early Carboniferous.