A dike swarm up to 15 mi. wide and at least 85 mi. long cuts northwestward across the Sierra Nevada, Inyo Range, and Argus Range in eastern California. The dike rocks range from lamprophyre to granodiorite porphyry, the more mafic types predominating. The dikes are probably of Cretaceous age, for they intrude some granitic plutons of the Cretaceous Sierra Nevada batholith but are truncated by others. The dikes in the Sierra Nevada were metamorphosed by the younger granitic intrusives. In the Inyo and Argus ranges the dikes are hydrothermally altered, but otherwise unmetamorphosed. Two sequences of granitic intrusion in the Sierra Nevada batholith (Mount Pinchot quadrangle) can be separated by the dike swarm. Each sequence apparently began with the emplacement of mafic granodiorite and concluded with alaskite; succeeding intrusions in each sequence were progressively more felsic. Two cycles of differentiation seem indicated. Chilled dike margins where the swarm cuts earlier plutons indicates that a significant time interval separated emplacement of pre-dike and post-dike plutons. The dike swarm shows no apparent offset where it crosses Owens Valley. This argues against suggested major strike-slip faulting along the line of Owens Valley since Cretaceous time.