Near Castle Rock, in Colorado, is quarried a soft, very light gray and pink stone of volcanic origin, which is commonly called lava stone. It is extremely light in weight, weighing only about no pounds per cubic foot, and can be cut with a knife. It weathers better than the soft sandstones, and its color makes it very suitable for rock face ashlar. It is difficult to obtain in large blocks, and is full of clay or air holes and often of invisible cracks, which render it dangerous for use in heavy buildings, but for dwellings it makes a very cheap, durable and pleasing stone. Owing to the small air holes which it contains it does not receive a finished surface, and is most effective when used rock face. There are a great many houses and several public buildings in Denver built of this stone. A similar stone occurs in the vicinity of the Las Vegas Hot Springs, and Albuquerque, New Mexico.
Blue Shale is a variety of sandstone that is dark blue in color, quite dense and hard, and makes a fair material for foundations. As a rule it does not work readily and often contains iron pyrites, which renders it unsuitable for ashlar or trimmings.
The only stone in many localities is a hard, igneous rock, called trap, which is suitable for foundations, but cannot be cut easily.
Such stones are only used for local purposes when no other can be obtained except at great expense.