Porphyry is a texture assigned to igneous rocks that contain larger grained crystals within a finer-grained aphanitic matrix. These crystals are usually feldspars or quartz and are called phenocrysts. The phenocrysts in this sample are orthoclase feldspar, also known as potassium feldspar. The formation of this type of rock is due to variable cooling rates of the magma. Initially, the magma cooled slowly deeper down in the mantle, allowing coarse crystals to form. As the rock rose more rapidly and broke to the surface, the fine-grained matrix was formed to surround the larger crystals as the cooling rate rapidly increased closer to the surface. The fine matrix and coarse phenocrysts are often of the same composition.