In the porphyritic facies of the granite, the feldspars make up the greater part of the groundmass. The crystals have so interfered with one another during the period of their formation that they have produced a hypidiomorphic, granular mass, through which are scattered the quartz grains in the same way as in the groundmass of a quartz-porphyry. At other times the quartz crystals have formed in the spaces between the feldspar grains. They then occasionally have some of their crystallographic faces preserved, indicating that they were formed before the complete crystallization of the feldspathic material; or they occupy the irregular spaces left between the feldspar grains, showing that they were formed last.

Chemical Composition. Analyses show that the typical granites of the district do not differ materially from typical granitic masses in other parts of the world. The degree of acidity often varies between rather wide limits. A complete analysis by Melville of the typical rock from Graniteville, which is out side of the area, gave the following results:

Another partial analysis of the granite from the vicinity of the Silver mines, by the St. Louis Sampling and Testing works, gave:

Mineralogical Constitution. The granite is a mixture of quartz and feldspar as the principal components, with some biotite also as an essential ingredient. As accessory consti

tuents there are apatite, zircon and magnetite. In order of crystallization the three minerals last mentioned are the first to form. They occur in minute crystals commonly with more or less well defined crystallographic faces. They appear as inclusions in all of the later formed minerals but most abundantly in the biotite.

Next in order of formation are the ferro-magnesian silicates, biotite and hornblende. Both of these minerals are comparatively rare. The first mentioned forms an important constituent only in a few cases. It is unevenly distributed, so that at some points the rock containing it is quite mottled, though only a short distance away the mica is entirely absent. Further, in a locality where black mica occurs it often forms segregation masses from a few inches to a foot in size. Although apparently made up of this mineral a close examination shows the presence of quartz and occasionally a small amount of feldspar. Sometimes the segregations are so large as to cause considerable annoyance in the quarrying.

Hornblende has been observed in a few instances but very sparingly. Of all the sections of the rock examined less. than half a dozen contain this mineral and then only as occasional crystals. Much of the dark colored component which is commonly called hornblende is not such, but black mica.

Both potash and soda feldspars are represented, the former constituting much the greater part of the rock. It is the reddish orthoclase and microcline that imparts to the granite its characteristic color. The triclinic feldspars are albite and oligoclase.

The quartz is usually allotriomorphic, with some gas bubbles and often slender hair-like needles.

Microscopical Features-The minute characters are of particular interest, and have an important bearing upon the investigation of the rock as a building stone. In thin slices, under the microscope, the granite is found to be an allotriomorphic or hypidiomorphic granular aggregate of quartz and feldspar, with some mica and the accessory constituents in minate crystals. Its general appearance is shown in the accompanying

photomicrograph (plate v, figure 1) which is magnified 30 diameters. The fundamental differences in the appearance of the granite and the porphyry is easily made out by comparing this illustration with the other one on the same plate (figure 2).

Structurally, two varieties of the granitic rocks are represented. First, there is the perfectly granular, in which the component grains are all more or less uniform in size, and in which the different periods of crystallization are not distinguishable. This is the typical granite. Second, there is the variety in which a greater or less disparity in the dimensions of the essential constituents is observable, and in which at least two generations of crystals may be readily made out. In its best development it is the porphyritic granite. The essential difference in these two facies of structure are more clearly brought out in the annexed cuts (figures 7 and 8):

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FIGURE 7. Thin slice of granite.

FIGURE 8. Thin slice of porphyritic granite.

The feldspars show little indications of decomposition, which is a very favorable point in determining the value of the rock for building purposes. Moreover, the absence or rarity of biotite and other ferro-magnesian compounds is another factor in showing the great resistance of the granite to atmospheric decay.

Several very interesting features are brought out through microscopical examination, which deserve passing mention. These are the occurrence of secondary enlargements of feldspar crystals which have suffered rounding through magmatic corrosion. The later feldspathic material is oriented the same