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222 GEOLOGY OF THE HIGH PLATEAUS. ^ 



bare its anatomy. Our surprise is often great at finding the cone wonder- 

 fully well stratified, but in a peculiar way. The most perfect stratification 

 is presented when the dissecting cut is made radially. But when a cut 

 transverse to the radius is made by excavations of another stream, the strati- 

 fication, though still conspicuous, is much less uniform and harmonious. 

 The cone appears to be built up of long radial or sectoral slabs superposed 

 like a series of shingles or thatches. 



There are marked difi^erences between the cones formed by streams 

 which have their entire descent within unaltered sedimentary strata and those 

 running among volcanic and metamorphic rocks. The fragments resulting 

 from the decay of sandstones, limestones, and shales are much more sus- 

 ceptible to the influence of weathering and are more readily worn-out by 

 the abrasion of travel. Even when they escape destruction by the wear 

 of the torrent and reach a resting-place upon the surface of the cone, 

 the gentler but more insidious action of meteoric forces gradually crum- 

 bles them to sand or dissolves them, and they at length disappear. But 

 the compact volcanic and metamorphic rocks are much more durable 

 "; and do not yield so readily either to mechanical or chemical forces ; more 



of them reach the cones, where they survive long enough to be buried 

 beneath later accumulations and thus receive final protection from dissolu- 

 tion. Hence the cones derived from the waste of sedimentary strata sel- 

 dom contain much coarse debris, while those from harder rocks are largely 

 composed of it. This diff'erence in texture in turn produces some difference 

 in the proportions of cones. The sedimentary cones are usually very 

 slightly flatter and broader. The difference in this respect is on the whole 

 quite small, but the measurement of a considerable number of both kinds 

 seems to indicate that it really exists. 



In consequence of the flatness of the cones and their lateral confluence, 

 the general result of their serial aggregation is a long and thick stratum 

 made up of many subordinate folia. In process of time it may also 

 become consolidated and hardened into a rock mass resembling in all 

 essential respects the stratified conglomerates usually reckoned among the 

 members of a, stratigraphic series. That distinctions between such a con- 

 glomerate and one deposited littorally would be readily detected after close 



