Beds of volcanic ash occur widely among recent deposits in the western United States.  In Nebraska ash beds are found in twenty counties, and are often as white as powdered pumice.  The beds grow thicker and coarser toward the southwestern part of the state, where their thickness sometimes reaches fifty feet.  In what direction would you look for the now extinct volcano whose explosive eruptions are thus recorded?

Tuff.  This is a convenient term designating any rock composed of volcanic fragments.  Coarse tuffs of angular fragments are called volcanic BRECIA, and when the fragments have been rounded and sorted by water the rock is termed a volcanic conglomerate.  Even when deposited in the open air, as on the slopes of a volcano, tuffs may be rudely bedded and their fragments more or less rounded, and unless marine shells or the remains of land plants and animals are found as fossils in them, there is often considerable difficulty in telling whether they were laid in water or in air.  In either case they soon become consolidated.  Chemical deposits from percolating waters fill the interstices, and the bed of loose fragments is cemented to hard rock.

The materials of which tuffs are composed are easily recognized as volcanic in their origin.  The fragments are more or less cellular, according to the degree to which they were distended with steam when in a molten state, and even in the finest dust one may see the glass or the crystals of lava from which it was derived.  Tuffs often contain VOCLANIC bombs,—­balls of lava which took shape while whirling in the air, and solidified before falling to the ground.

Ancient volcanic rocks.  It is in these materials and structures which we have described that volcanoes leave some of their most enduring records.  Even the volcanic rocks of the earliest geological ages, uplifted after long burial beneath the sea and exposed to view by deep erosion, are recognized and their history read despite the many changes which they may have undergone.  A sheet of ancient lava may be distinguished by its composition from the sediments among which it is imbedded.  The direction of its flow lines may be noted.  The cellular and slaggy surface where the pasty lava was distended by escaping steam is recognized by the amygdules which now fill the ancient steam blebs.  In a pile of successive sheets of lava each flow may be distinguished and its thickness measured; for the surface of each sheet is glassy and scoriaceous, while beneath its upper portions the lava of each flow is more dense and stony.  The length of time which elapsed before a sheet was buried beneath the materials of succeeding eruptions may be told by the amount of weathering which it had undergone, the depth of ancient soil—­now baked to solid rock—­upon it, and the erosion which it had suffered in the interval.

If the flow occurred from some submarine volcano, we may recognize the fact by the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers.