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IGNEOUS ROCKS.

ROCKS FROM THE QUITMAN MOUNTAINS.

A series of granitic rocks. They contain as dark constituents biotite, a brown or greenish primary amphibole, and a pale green pyroxene (diopside), and belong under the head of pyroxene amphibole granitite. Their alkali-feldspar shows frequently a very fine twinning lamination (anorthoclase?) or a microperthitic intergrowth of orthoclase and albite, as is the case in acid rocks, rich in soda and poor in lime. Besides, there is a plagioclase in various amounts, in some specimens distinct from the common feldspar by its different color. In accordance with the basic charracter of the rock, quartz is scarce; in some specimens the latter mineral is nearly wanting, the rocks thus forming transitions to syenites.

Granitic dike rocks accompanying the above mentioned abyssic rocks. These are aplites and granite porphyries, the later showing a considerable amount of plagioclase and only a little quartz.

Rocks resulting from limestones by contact metamorphism on abyssic rocks. According to von Streeruwitz the unaffected limestone belongs probably to the Cretaceous (Kreide), and the changed rocks are bordering the boundaries of the above mentioned granitic rocks. There is a complete series of specimens from unaltered limestone to rocks composed almost entirely of lime silicates, viz., garnet, vesuvianite, epidote, diopside, etc. Some of the specimens are nearly pure garnet rocks made up of green or brown garnet in grains and crystals (110), often a centimeter or more in size. The green garnet shows usually a strong abnormal double refraction. In other cases the limestone became only coarse crystalline.

A series of effusive rocks with phenocrysts of unstriated feldspar in a dense, felsitic ground mass of black, brown, or greenish color. Phenocrysts of quartz are wanting. Under the microscope the feldspar shows a microperthitic structure or a fine twinning lamination, as it is the case in the group of the anorthoclase feldspars. The ground mass is vitreous or microfelsitic. Chemical analysis of the rocks and feldspars must decide whether these rocks belong under the head of felsite porphyries or keratophyres.

Some altered basic rock specimens coming under the group of diabases and augite porphyries. One of the latter in a somewhat fresher condition shows phenocrysts of a lath-shaped plagioclase in a fine-grained gray ground mass. Under the microscope the rock is seen to be made up of phenocrysts of plagioclase, a green pyroxene, brown hornblende nearly entirely replaced by opaque magnetite grains, and a holocrystalline ground mass consisting of lath-shaped feldspar, small needles of pyroxene and some hornblende.