Age dating conducted on a group of volcanic rocks from §åi Bï area (Hoµ B×nh) by the Rb-Sr method yields a result of 283 ±21 Ma. This age corresponds to the dating of Phan Tr­êng ThÞ et al. (1974) based on fossils from limestone interbedded in effusives of the Suèi Rót area. According to that the volcanic activity in the S«ng §µ structure should be taken back to C₃ - P₁ instead of P₂ as believed by many geologists.