SUMMARYFollowing the deposition of Wutai formation, there occured a period of intense disturbance accompanied by igneous intrusion through which the Wutai formation has been strongly folded and metamorphosed. Since then ensued a long period of erosion until Mesozoic time when the folded mountains were completely reduced to a peneplane. It was due to subsequent warping and faulting of this old peneplaned surface that came into existence the Laiyang basin which extended at that time in NE‐SW direction. So far as the present exposures are concerned, it began from Tieh Kou Tsion the northeast and thence southwestward through the Lai Yang city to Chang Ying of Chiao Hsien on the southwest. Like most of the Mesozoic basins in other parts of the country, its sediments including the fossils as well were of a continental type, accumulated probably under a dominantly moist and warm climate. The rocks composing the floor are usually of the very ancient type, ranging from Archaean to Algonkian; they are as a rule rather complicated in structure. The Laiyang basin was then closed by the explosion of the Ching Shan volcanics which were in turn succeeded by the Wangshih series, the last record in Mesozoic time. After the deposition of the last Cretaceous sediment, warping as well as faulting became active again in Tertiary time. Some of them are still very conspicuous in the present topographical feature