The combination of orthoclase feldspar (or microcline), quartz, and either mica, hornblende or augite is termed granite, if the texture is coarse enough so the individual minerals can be recognized with the unaided eye. The rock is light-colored because the feldspar and quartz dominate. Accessory minerals may be present such as apatite, zircon, beryl or magnetite. Varieties of granite are distinguished according to the dark mineral present. When this is muscovite, it is a muscovite-granite; when it is biotite, a biotite-granite; if it is hornblende, a hornblende-granite; etc. The size of crystals in granite varies widely. When they are as small as ¹/₁₂ of an inch in diameter, it is termed fine grained; from ¹/₁₂ to ¼ of an inch, it is medium-grained; when larger, it is coarse-grained. In some cases the crystals may be over a foot in diameter which is known as giant granite.
Originally granite was a great mass of molten magma, which has cooled very slowly, having been intruded or thrust up in great stocks or batholiths beneath overlying rocks, which acted as a blanket to prevent rapid cooling. These overlying rocks, in their turn, have been acted upon by the heat and metamorphosed. Granite is particularly likely to have been formed under mountain folds; so that, after the mountains have been more or less completely eroded away, the great masses of granite have come to the surface to mark the axes of the ranges; and even after the mountains have been wholly worn away, the granite remains to mark the sites on which they stood.
In the granite mass itself, there are often veins and dikes, which probably resulted from the shrinkage of the cooling granite, and they are filled with a different and usually coarser granite known as pegmatite. This pegmatite formed from the residual magmatic material, so that as some of the elements had already crystallized out, the granite in these dikes is of different composition. The extreme coarseness of these pegmatites seems to be due to the character of the mineralizing agents left in the dikes. In some of these pegmatites the feldspar and quartz are so intergrown, that when broken along the cleavage surface of the feldspar, the quartz appears like cuneiform characters, and this variety has been given the name graphic granite (See [Plate 53]).
When granite is exposed to weathering, the feldspar is the first mineral to be decomposed, altering eventually into carbonates, quartz and kaolin. The dark minerals are only slightly less susceptible and they break down into carbonates, iron oxides and kaolin. The original quartz remains unchanged. Of these products the carbonates, some of the iron oxide and a little of the quartz are carried away in solution. The kaolin and some of the iron oxide is in fine particles and they are carried by the water until it comes to the lakes or the sea. The quartz is left in coarser grains, which are more slowly transported, and deposited in coarser or finer sand and gravel beds.
Granites are widely used for building stone, because they can be worked readily in all directions, and have great strength and beauty. The color depends largely on the color of the feldspar, which may be white or pink, in which case the granite will be gray to pink.
Granites occur throughout New England, the Piedmont Plateau, the Lake Superior Region, the Black Hills, Rocky Mountains, Sierra Nevada, etc.
[Syenite]
[Pl. 54]
The combination of orthoclase and either mica, hornblende, or augite is syenite, the texture being coarse enough so that the individual minerals can be distinguished by the unaided eye. It differs from granite in the absence of quartz. Syenite is a light-colored rock with the feldspar predominating. Minerals like apatite, zircon, or magnetite may occur in it, as accessory minerals. The foregoing would be an ideal syenite, but usually there is some plagioclase feldspar also present. If this occurs in such quantities as to nearly equal the orthoclase feldspar, the rock is termed a monzonite; if it predominates, the rock becomes a diorite. The presence of quartz would make this rock into a granite. Such a compound rock has its type form, and when the proportions of the component minerals are changed, it grades into other types.
Like the granite, syenite is an intrusive rock, which occurs in stocks and batholiths along the axes of present or past mountain ranges. The original magma welled up under the mountain folds, where it cooled slowly, metamorphosing the adjacent rocks. Like granite it has only been exposed after a long period of erosion has removed the overlying layers of rock.
Syenites are not as abundant as granites, but they occur in the White Mountains, near Little Rock, Ark., in Custer Co., Colo., etc.