These rocks are formed out of the fragmental materials ejected from volcanoes. The materials are of course igneous, but the rocks themselves differ from the typical igneous rocks in several important respects. They have not been formed in their present state of aggregation by cooling from a molten mass, and in many cases they are more or less distinctly stratified. It seems best, therefore, to group them separately, under the name pyroclastic.

Volcanic Agglomerate, or Breccia, is a mass of angular blocks of lava, with which may be mingled fragments of sedimentary rocks, which the volcano has torn off from the sides of its chimney. The blocks may be loose or cemented together into hard rock by a filling of finer materials. Ordinarily the breccia is formed only near the vent, but sometimes it is developed on a great scale, as in the eastern part of the Yellowstone Park.

Tuffs are masses of volcanic ashes and dust, which accumulate in beds, either on the land or in bodies of water. Even in falling through the air, the particles are sorted, in some degree, in accordance with their size, and the tuffs are thus usually stratified, and frequently have fossils in them. When accumulated under water, the ashes are, of course, stratified and may be mingled with more or less sedimentary debris. Such subaqueous tuffs pass into the ordinary sedimentary rocks, by the gradual diminution of the volcanic material. When examined under the microscope, even the finest tuffs are found to consist of crystals and particles of glass.

The volcanic breccias and tuffs may best be classified in accordance with the nature of the component fragments. Thus, we find rhyolite tuffs and breccias, andesite tuffs and breccias, basaltic tuffs and breccias, and the like.