ENDUCH.] ERUPTIVE EOCKS. 99 



portions. In most instances of these kinds the openings were found to 

 be larger than auy portion of the interior. 



Erosion by flowing water is productive of approximately the same re- 

 sults in these two groups, having a greater effect iu the latter. Valleys 

 of erosion, occurriug in a series of "soft" flows, are wider and slope 

 more gently at the sides than the others. In consequence of the uu- 

 brokeu flows, which often extend over many square miles, water collects 

 in slight depressions, which are incident to the gentle dip or due to 

 other causes. In this way numerous lakelets are sometimes scattered 

 over plateaus, which latter are the favorite localities for such occur- 

 rences. Where either the shallow basins existing are not of sufficient 

 depth to contain a body of water, or the quantity of the latter is too 

 small to fill them, extensive swamps are formed on the summits of pla- 

 teaus. Vegetation, wherever the conditions may be favorable to its ex- 

 istence, has a great influence in shaping minor details of orographic 

 leatures. Many of the varieties of volcanic rocks decompose very slowly, 

 and furnish but little soil for vegetation. This is particularly true of arid 

 regions. Vegetation and moisture combined, however, rapidly break 

 down the first barrier of impenetrability, and then the process of disin- 

 tegration proceeds steadily. Besides the lack of water, the numerous 

 avalanches along the slopes of mountains prevent, to a great extent, the 

 growth of vegetation. Avalanches — composed mainly of bowlders and 

 detritus, less of snow — form a very prominent feature in all those dis- 

 tricts where the volcanic strata exhibit a columnar structure, and where 

 hard, brittle beds are overlying softer ones, more readily eroded. The 

 steeper the mountain-slope is, ihe farther can the avalanche descend. 

 It destroys, in its course, what vegetation may exist, and by being devoid 

 of soil prevents the springing up of a new growth. 



Erosiou by glaciers is in proportion to the harder or softer nature of 

 the strata over which they pass, la the one instance they produce but 

 slight modifications of the surface; in the other the effect produced by 

 them aids more decidedly in changing the detail-orographic features. 

 Sand, driven by wind, has considerable influence upon the minor details 

 of mountain-slopes and isolated groups, carving its peculiar rounded 

 indentations into such portions of the strata as may be found to be 

 sufficiently' yielding. 



Besides these two groups of the massive flow volcanics, we have the 

 tuff's. In their behavior they closely resemble marls. Loosely cemented, 

 they offer scarcely any resistance to erosive agents, and are cut into 

 every variety of picturesque forms. Cappings of solid volcanic material 

 may protect them in places, but wherever they are exposed they exhibit 

 the most grotesque groups imaginable. Creeks and rivers cut deeply 

 into banks that are composed of this material, and by thus undermin- 

 ing superincumbent strata, cause a vertical or approximately vertical 

 droppingof the latter. Tuff's are of local occurrence, but may be counted 

 upon as being met with in tlie same horizon within a circumscribed area. 

 Conglomerates are not wanting in volcanic areas ; show the same char- 

 acteristics, however, as any others. Conglomerates of sedimentary 

 formations have been discussed above, and what has been said of those 

 is also applicable here. 



Local massive erw^ftows.— These differ from the preceding group in 

 several respects. They do not extend over very large areas, and are 

 not so regularly stratified. As a rule, isolated mountaiu-groups are thus 

 formed that have no direct connection or relation to any of the existing 

 chains or ranges. Volcanic material has been ejected through one or 

 more fissures or tubes, and for certain mechanical reasons has not spread 



