Analysis of geologic data from 54 localities, mainly in the western United States, shows that concordant igneous masses intruded zones in nearly flat-lying sedimentary rocks where the thickness of cover was 3000 to 7500 feet. The depth of intrusion was apparently affected by a well-defined parting surface (bedding plane or unconformity), static load of the overburden (lithostatic pressure), and presence of a fluid barrier above the intrusion.
Most bodies intruded along well-defined bedding planes in sedimentary rocks and flow surfaces in extrusive igneous rocks; some intruded along unconformities. The range in overburden pressure is about 3000 to 7500 psi for intrusions at depths of 3000 to 7500 feet. Magma pressure for lateral injection at the lower limit of the depth range, therefore, must have exceeded 7500 psi to lift 7500 feet of overburden. A fluid barrier, such as a shale, overlies almost all intrusions studied and tended to retard upward advance of magma. At depths less than 3000 feet, the barrier shales may be brittle and easily ruptured by the fluid pressure, permitting steam and magma to escape to the surface. Below 3000 feet the shales may become ductile.