The Sierra de Tamaulipas is an area of Cretaceous carbonate sediments of some 50x70 km overlain and intruded by a broad series of igneous rocks of alkaline and transitional character. Detailed maps of the igneous rocks are not available but numerous analyses and brief petrographic descriptions are given by Robin (1982, Annexe). Most of the igneous rocks, which include rhyolites, trachytes, phonolites, nepheline syenites, basalts, basanites and nephelinites, are Oligocene-Miocene in age, but there is also a later series of Pliocene-Quaternary trachytes, phonolites and basalts. The rhyolites usualy form domes and include comenditic varieties with aegirine. Trachytes commonly form sills but are also present in necks and as microsyenite in laccolithic intrusions. They range from quartz-bearing varieties to trachyphonolites and are commonly aegirine-phyric, while fayalite and sodic amphibole is present in some varieties. There is a continuous gradation to phonolites and nepheline syenites. The nepheline syenites contain abundant aegirine, together with hedenbergite, biotite and sodic amphiboles; aenigmatite, eudialyte and lavenite have also been identified. Phonolites are similar but only rarely contain amphibole. The basaltic rocks include necks of augite-phyric dolerite with a little quartz and these rocks are considered to be chemically transitional between alkaline and tholeiitic types. On the western and southern flanks of the area are scattered remnants of basanite and nephelinite lavas. The nephelinites may contain biotite phenocrysts and pyroxenite and peridotite nodules have been found in rocks of this group (Robin, 1982, p. 301). The Pliocene-Quaternary trachytes and phonolites form extensive flows and the basalts of this group are alkali olivine-bearing varieties.