On the mountain slopes, of course, the same seasonal patterns of heat and precipitation occur, but they are tempered or enhanced by elevation. With increasing elevation, temperatures decline and precipitation increases. When July temperatures at headquarters are averaging in the nineties, they will be in the sixties on top of the Rincons. In the cactus forest at the foot of Tanque Verde Ridge only about 10 to 12 inches of rain fall in a year, while Manning Camp, at 8,000 feet, gets about 18 to 25 inches. These up-slope gradients of temperature and precipitation, as we shall see, have pronounced effects on plant and animal life. [Illustration: Gila monster.] [Illustration: Banded augen gneiss on Cactus Forest Drive. Javelinas formerly took refuge from the sun in the dens under these overhanging rocks.] rocks: foundation and soilmakers Soils are derived from the fragmentation and decomposition of rocks. Combinations of soils and climate, varying from place to place, create an almost infinite number of environments with differences so subtle and small as to make it appropriate to refer to them as microenvironments. Though these differences be minor, they are often sufficient to create niches each of which becomes a habitat for a particular group of plants and animals. Thus it is that the saguaros and other desert vegetation in Saguaro National Monument and vicinity are found in particular environmental or ecological niches where just the right combinations of soils, moisture, sunlight, temperature and other factors are present. Since the rocks provide the foundation and the source of soils that support the plant and animal life, it is helpful to understand something about the origin and evolution of the rocks and the landscape of which they are a part. Clues to this story are to be found in the composition of the rocks and their relationships to one another. Rocks of the three major classes—igneous, metamorphic, and sedimentary—are found within the monument. The igneous rocks include granites and various kinds of lava flows, together with some intrusive dikes and veins. The metamorphic rocks are represented by gneiss and schist. Lastly, sedimentary rocks occur as limestone, sandstone, and alluvial fill material. Each of these great classes of rocks was formed in a different manner, and these differences reveal the nature of the events which are a part of the geologic history of the monument and surrounding area.