Basaltic rocks in the Umpqua formation
INTRODUCTION In 1930, during a study of the geology of an area in west-central Oregon within which occur the quicksilver deposits of Blackbutte, Elk-head, Bonanza, and Nonpareil, a series of extrusive and intrusive basaltic igneous rocks were encountered. Part of this area lies within the Roseburg quadrangle, mapped by Diller in 1898. Diller called all these rocks “diabase.” 1 It is the purpose of this paper to show that the basaltic rocks include amygdaloidal and ellipsoidal basalt flows, olivine basalt dikes, norite sills and necks, and hypersthene-augite basalt dikes and that they represent more than one period of igneous activity. SUMMARY OF GEOLOGY A full discussion of the geology of this area is found in a recent bulletin of the United States Geological Survey,2 it can be briefly summarized as follows: The southwestern part of the area (Fig. 1) is underlain by the Umpqua formation, a series of interbedded sandstones . . .