A terrane of 1,350- to 1,400-m.y.-old silicic volcanic and plutonic rocks in the buried Proterozoic of the mid-continent and in the Wet Mountains, Colorado.

A terrain of granite and rhyolite underlies a large region extending from central Kansas south to the Llano terrain of central Texas and from E Colorado to W Missouri. Rocks within this region are remarkably uniform in petrographic character and range 1350- 1400 m.y. in age. These rocks are distinctly younger than similar rocks with zircon age of 1450-1480 m.y. in the St. Francois Mt of SE Missouri and in the buried basement of Illinois, Indiana and eastern Ohio. Thus similar rocks were formed in distinctive parts of the mid-continent during two episodes. The San Isabel batholith of the Wet Mt, Colorado, emplaced at moderate depth in approx 1700 m.y.- old crust yielded an age of 1360 m.y. and is thus coeval with the granite and rhyolite of the southern mid-continent region. A possible model is that these mid-continent rocks are essentially a veneer emplaced upon and within the older, possibly 1700 m.y.-old crust; the San Isabel batholith is a deep-seated representative exposed because of crustal uplift. It is implied that most of the mid-continent may also consist of older, 1700 m.y.-old crust beneath the veneer of younger volcanics and epizonal plutons.-L.di H.