The metamorphic rocks are the most difficult of all the rocks to determine and understand, because the amount of change through which they have gone is greatest, but for this same reason they offer the most interest, for the agents which caused the changes are of the most dramatic type of any that occur in Nature. From one place to another a single layer of metamorphic rock changes according to the greater or less heat to which it was subjected, making a series of related rocks of the same composition but with varied amount of alteration. For this reason in naming metamorphic rocks, a type is named, and from that there will be gradations in one or more directions, both according to composition, and according to amount of heat involved. If it is possible to follow a given layer of metamorphic rock from one place to another this is of great interest; for by this means, many variations in the type will be found, both those resulting from a different amount of heat, and those due to the local changes in the composition of the original rock. One further consideration has to be kept in mind. When a rock is metamorphosed the high temperatures either drive off all water, or the water may be used up in the making of some of the complex minerals. When such a metamorphic rock later comes near the surface and is exposed to the presence of ground water, and that leaching down from the surface into the rocks, several of the minerals formed at high temperatures will take up this water and make new minerals such as serpentine, chlorite, etc. They are always associated with metamorphic rocks, and have been metamorphic rocks, but since then have become hydrated, forming minerals not at all characteristic of high temperature. The following shows the relation of the sedimentary and igneous rocks to their metamorphic equivalents. _Loose sediment_ _Consolidated sediment_ _Metamorphic equivalent_ gravel conglomerate gneiss sand (quartz) sandstone quartzite mud (sand and clay) shale schist clay shale slate or phyllite marl limestone marble peat bituminous coal anthracite to graphite coarse igneous rocks such as gneiss granite, syenite, etc. fine igneous rocks such as schist trachite, rhyolite, etc.