[Illustration: Figure 1. Igneous dike cutting the Pinney Hollow formation. The dike is the darker rock which trends toward the upper left of the photograph. This dike is located approximately 100 yards south of the Pinney Hollow Historical Monument and on the east side of State Highway 100A.]
The third basic principle to command is known as the Law of Faunal Succession. In generalized form this law states that each stratum of rock contains its own distinct group of animal or plant remains, termed fossils, and that these same remains can be recognized throughout the world wherever they occur. Since plants and animals changed through time and because their remains are found throughout the world, it is possible to erect a worldwide time scale based upon animal and plant evolution. In short, the fossils found in particular rock are characteristic representatives of the life at the time that rock originated and the fossils found could have been entombed only at that time. One stratum, therefore, would have a total fossil assemblage quite different from the stratum above or below. Without this time reference chart it would be impossible to reconstruct what did happen during any one time interval in the past.
In addition to these three basic laws it is necessary to mention the rudiments of rock classification. The geologist divides _rocks_[1] into three major groups which are termed Igneous, Sedimentary and Metamorphic rocks. Igneous rocks are those formed by the solidification of molten material. This molten material was thrust into the outer crust of the earth from below and after cooling became a solid igneous rock such as granite, or in other cases it flowed out over the surface of the earth in the form of volcanic lava. Some small igneous bodies, termed dikes, can readily be seen along State Highway 100A adjacent to Calvin Coolidge State Forest Park (See photograph, Fig. 1).
Sedimentary rocks are formed in quite a different manner and differ in general appearance. These are what might be considered second-hand rocks. They are composed of particles derived from other rocks, igneous, metamorphic or older sedimentary, which have been carried by streams, wind or ice to a place of rest and there cemented into rock. Perhaps you can visualize a river which, throughout its course, runs over rocks of many types. This river would pick up particles of rock from its bed and banks and transport these to a lake or perhaps the sea, where the various transported materials would settle to the bottom in distinct layers. The first layer deposited would become buried under thousands of tons of overlying layers of sediment whose weight and resultant pressure, together with the presence of adequate rock-cementing material such as calcium carbonate or silica, would cause the bottom layer to harden into rock. The layered appearance of sedimentary rocks is one of their most characteristic features and these rocks are said to be bedded or composed of many individual beds of sedimentary rock. Sandstone, composed of sand size particles; shale, originally mud; and limestone, once lime-rich mud, are examples of sedimentary rocks.