On this map, the relative strength of the magnetic field is indicated by blue contours. These are plotted on top of geologic , topographic, and tectonic information. At this map scale, the strongly magnetic rocks in the subsurface show up as closely packed blue contours. As discussed earlier, these highly magnetic rocks are probably iron skarn and gabbro, both of which contain many magnetic iron-rich minerals. Gabbros and skarns are exposed at the surface in a couple of places, but mostly are concealed by other kinds of rocks.
Some granites are relatively nonmagnetic too and also produce smooth magnetic contours.
These areas are underlain by granite containing very few iron-bearing minerals. Linear gradients in the magnetic field match mapped faults in several places, but not in others. Mapped faults without a coinciding magnetic lineament can be explained in several ways. The fault may have a very small displacement, or the fault may have a large displacement which has brought rocks together on each side of the fault with similar magnetic properties. In either case, the fault would be difficult to resolve in the magnetic contours.