75 samples of 7 rock types have been analyzed for magnetite content by magnetic separation and chemical analysis, making allowance for iron which occurs as ilmenite, hematite, and silicates. Magnetic susceptibility shows a clear dependence on magnetite content but with too much scatter to permit prediction of one from the other. For small magnetite content V (in volume percent), susceptibility k is roughly given by k = 3,000X10 (super -6) V cgs. Based on 200 outcrops of 11 rock types, the approximate mean susceptibility of basalt and diabase is 2,500X10 (super -6) , of rhyolite and gabbro 1,000X10 (super -6) , of acid intrusives including granite 400X10 (super -6) , of greenstone 100X10 (super -6) , and of slates 50X10 (super -6) . Variability is high for acid intrusives, intermediate for basalt, rhyolite, and greenstone, and lower for diabase, gabbro, and slate.