FRAGMENTAL VOLCANIC EOCKS— 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. — 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 mate- 

 rials 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, feld- 

 spar, 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 



