Tuff, a term not to be confused with tufa on page 215, is the name used to designate the finer fragmental ejecta of volcanic eruptions, which are also often referred to as “volcanic ash,” but the word, ash, conveys the false impression that the rock is a remnant of something burned, and is therefore not a good term. When first ejected, tuff is loose material, but it is usually soon cemented to make a more or less firm mass of rock, for which the term, tuff, is still retained. In some cases, while still loose, it is carried by streams to a distance and deposited in more or less sorted and layered beds: and the finer tuff is often carried by the winds and laid down, at a considerable distance from its source, in so called “ash beds.” In both these cases, sedimentary characteristics have been added to the tuff, and layering which is characteristic of sedimentary deposits, is present. These transported tuff beds are really sedimentary, but as there is little change in the material, they are referred to here and not again. These tuff beds are not at all uncommon in the sedimentary deposits of Tertiary age in the Rocky Mountain region. The coarser material of volcanic eruptions usually goes under the head of breccia. Breccia This term is used to describe the coarse fragmental ejecta of volcanic eruptions. It is also used, in the section under sedimentary rocks, in a broad sense to include all angular unworn fragmental material, whether of igneous or sedimentary origin. For this reason, when dealing with igneous rocks, it is usual to designate the fragments according to their composition, making such terms as _trachite-breccia_, _rhyolite-breccia_, etc. While still loose (and also even when cemented into beds of rock), it is customary to designate the smaller fragments, from the size of a grain of wheat up to an inch or two in diameter, as _lapilli_; the larger fragments, from two inches up to a foot or so in diameter, as _bombs_; and the largest masses, often tons in weight, as _volcanic blocks_. Obsidian Pl. 55