[2]A fault is a break or fracture in the rocks of the earth’s crust where one side of the fracture moves in some direction relative to the other side. Some recent earthquakes in California and Nevada have produced visible faults where, for instance, roads were cut and one side displaced several yards from the other. [3]Most of the mineral grains which make up a schist, a metamorphosed shale or sandy shale, can be seen with the naked eye, and the parallel to subparallel arrangement of platy or flaky minerals, such as the abundant mica, is the most obvious feature. In bright sunlight a schist presents a very shiny, almost silvery appearance which is caused by the reflection of sunlight from the many parallel mica plates. A phyllite has much the same mineral composition as that of a schist; however, the individual mineral grains are too small to be seen without the aid of a magnifying glass. As with the schist, a phyllite when viewed in direct sunlight, appears shiny. Quartzites are metamorphosed sandstones which have been fused into great hardness through the application of heat and pressure. [4]A geologic formation consists of a group of similar rocks, or sequence of rocks, which can be easily distinguished from adjacent rocks and whose distribution can be plotted on a map. [5]More technically this schist is termed a chlorite-sericite-quartz schist. [6]The geologist divides geologic time into four major sections, designated Eras. These Eras are in turn subdivided into Geologic Periods which are of shorter time duration. The Cambrian Period is the oldest of the Paleozoic Era and began some 550 million years ago. The Ordovician Period is the next oldest of the Paleozoic Era and started about 440 million years ago and ended close to 360 million years ago. Transcriber’s Notes —Silently corrected a few typos. —Retained publication information from the printed edition: this eBook is public-domain in the country of publication. —In the text versions only, text in italics is delimited by _underscores_. End of the Project Gutenberg EBook of The Geology of Calvin Coolidge State Forest, by Harry W. Dodge, Jr.