Plagioclase feldspar with any one of the pyroxenes, most commonly augite, is gabbro. There is a wide range in the relative proportions of the two minerals making gabbro. At one extreme are rocks made entirely, or almost entirely, of plagioclase feldspar, which are known as anorthosites, and occur in parts of the higher mountains of the Adirondacks like Mt. Marcy, in several places in eastern Canada, etc. Then there are the typical gabbros where the feldspar and augite are more or less equally represented. At the other extreme come those gabbros in which the pyroxene predominates, in the most marked cases the feldspar being entirely lacking, and the rock being termed a pyroxenite. When the pyroxene of a gabbro is either enstatite or hyposthene (usually the latter) the gabbro is often called norite. Magnetite, biotite, and hornblende may occur in small quantities as accessory minerals. Gabbro is a common intrusive rock, occurring in stocks, batholiths, and dikes, and often varies considerably in different parts of the mass. Like granite the mass solidified far below the surface, under some mountain fold, and has only been exposed as the result of weathering away the layers of overlying rock. Gabbros appear much like diorites, but are distinguished by the fact that the dark mineral is one of the pyroxenes, instead of an amphibole or a mica. They are widely distributed, being found in the White Mountains, near Peekskill, N. Y., Baltimore, Md., about Lake Superior, in Wyoming, the Rocky Mts., etc. Peridotite A rock made up of olivine and augite (or any of the pyroxenes) is peridotite. As it contains no feldspar, and both augite and olivine are dark-green to black in color, these rocks are always dark green to black in color and of considerable weight. They are usually rather coarsely crystalline. Peridotite is usually associated with gabbro, making dikes which lead from the main gabbro mass. Less frequently it occurs independently, making up an intrusive mass. Hornblende and mica may be present in small quantities, as accessory minerals. In general these are rather rare rocks, making dikes connected with stocks or batholiths of gabbro. Peridotite is found near Baltimore, Md., in Custer Co., Colo., in Kentucky, etc. Pyroxenite