The combination of plagioclase feldspar, quartz and either mica or hornblende makes quartz-diorite, sometimes called _tonalite_. The above would be the typical quartz-diorite, but there is usually some orthoclase present, which if it equals the plagioclase feldspar in amount makes this into a monzonite; or if it dominates, it makes the rock a granite. Quartz-diorite is darker colored than the two preceding rocks, the dark minerals being about as abundant as the light-colored ones, such as feldspar and quartz. For this reason the weight is also somewhat greater. Like the others this is an intrusive rock, occurring in stocks and batholiths, and indicative of great molten masses thrust up under mountain folds, and only exposed after the overlying rocks have been weathered away. It is by no means an abundant type of rock, but occurs at Belchertown, Mass., Peekskill, N. Y., in the Yellowstone Park, etc. Diorite Plagioclase feldspar with hornblende or mica, or with both, is known as diorite. It is distinguished from quartz-diorite by the absence of quartz. There is generally some augite in it, but if this should be equal to, or exceed the hornblende, the rock is then a gabbro. There may also be a small amount of orthoclase present, without taking this rock out of the diorite class, but if the orthoclase feldspar becomes dominant, then the rock is a syenite. Thus there is gradation into other groups in all directions. Apatite, magnetite, zircon, and titanite often occur in small quantities as accessory minerals. Generally the hornblende is in excess of the feldspar, so that the rock is a dark-colored one. Diorites occur in much the same manner as granites, being in stocks, batholiths or dikes, and are often associated with granites and gabbros. They are great intruded masses, associated with mountain making, and like the preceding rocks, cooled far below the surface, and have been exposed only after great thicknesses of overlying rocks have been weathered away. Peekskill, N. Y., the Sudbury nickel district in Canada, Mt. Davidson above the Comstock Lode in Nevada, etc., are typical localities for finding diorite. Olivine-Gabbro The combination of plagioclase feldspar with augite (or any of the pyroxenes) and olivine makes olivine-gabbro. The feldspar is usually one of those with considerable calcium in it, like labradorite; and as the dark minerals predominate, the rock is dark-colored. It is an intrusive rock, usually in dikes or stocks, where it solidified far below the surface, and was only exposed after the overlying rocks were weathered off. It is by no means an abundant type of rock, but is found in the Lake Superior Region, and near Birch Lake, Minn. Gabbro Pl. 54