This report represents part of a larger study undertaken to interpret the gross features of the earth's crust by aeromagnetic methods. The larger survey covers a 100-mile-wide strip along a great circle arc from Washington, D.C., to San Francisco, California. The area considered extends from about 200 miles east of the Rocky Mountains to 150 miles west of the coast of the Pacific Ocean. Inferences on gross structure and lithology are made from the magnetic patterns and trends shown on the map.

At the continental margin, the magnetic data strongly suggest that the oceanic crust becomes much more deeply buried as the continent is approached.

A large magnetic anomaly extending northwestward along the center of the Great Valley is probably caused by mafic intrusive rocks.

Broad wavelength magnetic anomalies over the Sierra Nevada are consistent with results obtained from gravity and seismic observations, indicating the batholith is underlain by gabbroic rocks.

The Basin and Range province is characterized by an east-west and northeasterly magnetic grain, whereas the structural grain is nearly north-south. A complex belt of circular and elliptical anomalies extends southwesterly from the Utah line south of Wendover to a point beyond Ely, Nevada. These anomalies are due to magnetic basement rocks and small intrusive bodies that extend diagonally across several ranges. From the Wasatch fault block to the Rocky Mountain area the predominant grain is conspicuously east-west. The trend becomes more northwesterly as one progresses from north to south. The aeromagnetic data, supported by geologic mapping, suggest the existence of a regional pattern of en echelon northeast-trending shear zones across the Rocky Mountains and Denver basin.

East-west magnetic lineaments, some as long as 400 miles, may represent fundamental fractures or fracture zones, most of which originate deep in the earth's crust. These fracture zones may be genetically related to the system of east-west strike slip faults which have been discovered beneath the oceans by various oceanographic institutions.

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