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Show FRAGMENTAL VOLCANIC ROCKSâ€"TUFAS. 71 which are, ordinarily, fairly distinguished from each other, though sometimes we find transition varieties connecting them. The first are the finer clastic beds, which are usually termed tufas or tuffs; the second are the coarser beds, generally termed volcanic conglomerates. I. Tufaceous deposits.â€"It has been noted of most of the volcanic regions of the world, where the period of activity reaches backward well into Tertiary time, that the earliest material erupted is seen in the present form of arenaceous or fragmental deposits. The finer or tufaceous beds have by many geologists been regarded as consisting of material blown out in a pulverulent form, and which, gathering into the drainage channels, was swept into neighboring bodies of water or descended there directly, and was stratified after the manner of sand or silt. Thus they infer that the volcanic activity in such regions was opened by the discharge of fragmental materials or " volcanic ashes," which, projected upwards, were wafted by the winds and precipitated over the adjoining country or waters. This view will be discussed further on. There can be no question that the most ancient volcanic materials hitherto distinguished in the District of the High Plateaus, and of which the relative age can be assigned, are certain sandstones or beds composed of exceedingly fine particles of shattered or rounded quartz crystals, feldspar, hornblende, and mica commingled in a base of amorphous matter, which is chiefly argillaceous or kaolinic and charged with oxides of iron. Wherever the grains are large enough to show their characters or have a gravelly consistency, they exhibit very clearly minute fragments of volcanic rock in a decayed or carious condition, resulting from the prolonged action of water and the atmosphere, and also show extreme mechanical attrition. This serves to distinguish them from ordinary sandstones, which are usually composed of rounded quartz-grains. In the tufas quartz-grains occur in insignificant proportions, and in their place we find granules of the complex but very massive and obdurate volcanic rocks. Fragments of hornblende and mica also occur, sometimes in great abundance. The condition of the ferruginous matter in the tufas is also very different in most cases from its condition in ordinary sedimentary beds. In the latter rocks it is usually present as a peroxide, sometimes hydrated, sometimes not. In the tufas it |