Twelve whole-rock and mica ages have been determined on igneous and metamorphic boulders from the Haymond Formation in the Marathon Basin. Ages range from 370 to 410 m.y., indicating that these rocks were formed during the period from Middle Devonian to Late Silurian. The granitic gneisses and metarhyolitic rocks have distinctive petrographic and chemical characteristics which suggest that the rocks were derived from the partial melting of clastic sediments. This is supported by the relatively high initial Sr 87/86 ratio of 0.713. The petrographic and isotopic evidence supports the following conclusions: (1) a geosyncline to the southeast of the Marathon depositional basin probably reached maximum subsidence in Silurian-Devonian time. (2) the partial melting in the downwarped geosyncline and resulting volcanism offers a source for the silica in the Caballos Novaculite, (3) the igneous and metamorphic activity in the geosyncline was followed closely by mountain building which supplied the source for the flysch sequence in the Marathon depositional basin. The mountains formed during this interval are named the Llanoria Mountains, and the orogeny causing the uplift is called the Llanorian Orogeny. Ten samples were dated from Texas and Mexico in an unsuccessful search for the specific source area for the boulders.