98 REPORT UNITED STATES GEOLOGICAL SURVEY. 



The primary tendency of strata belonging to this group is, to form 

 table-lands. Extending in long-continued flows over softer, less homo- 

 geneous rocks underneath, they present a solid, unbroken sheet, which 

 is less readily attacked by erosive agents than other formations would 

 be. Slight variations in dip direct the water-courses, or they are deter- 

 mined by favorable contact-lines with older groups, or follow fissures and 

 subsidences produced by subsequent seismic activity. A more or less 

 homogeneous paste contains several species of segregated minerals in the 

 rocks of this group, and it is the decomposition of these latter that ini- 

 tiates disintegration of the entire mass. Generally the action of atmos- 

 pheric agents in changing the surface of such areas is an exceedingly slow 

 one,and would bestillmore so,did uotexistingfracturelinesacceleratethe 

 same. The eventual result, as produced by the combined influence of 

 already -formed fissures and by erosion of all kinds, is a double one, de- 

 pendent, in a great measure, upon the thickness of the volcanic beds. 

 Wherever they are thin, overlying either crystalline or sedimentary 

 rocks, they form more or less evenly sloping plateaus or high lands. In 

 that case they occupy the highest portions of the region, rising at times 

 to isolated hills or mountains which denote the locality of the original 

 outflow. This is entirely changed, however, when we have a thickness 

 of volcanic beds amounting to 4,000 to 7,000 feet. Then we find that 

 ■erosion has produced narrow valleys, bordered on either side by moun- 

 tains. Gentle or steep slopes occur in the latter according to the physi- 

 cal character of the material composing them. As in the sandstone 

 areas, we here find that certain strata are harder than others, probably 

 hy having been reheated, and they then give rise to the formation of 

 *' benches" on the mountain or hill side. 



A homogeneous series of flows will produce a regular, pyramid-shaped 

 mountain, the slopes of which show horizontal stratification. Between 

 such mountains the valleys cut down at even inclinations of their sides, 

 unless caiions of separation should set in and produce the characteristic 

 vertical walls. One feature, occurring quite frequently on a grand 

 scale, changes all these results, however. It is columnar structure of the 

 volcanic beds. In that case the easy separation of columns, produced 

 by the action of gradual disintegration and, more particularly, frost, 

 ■causes them to fall from the face of the mountain and leave vertical 

 jirecipices, which, by a continuation of this action, slowly recede toward 

 the summit of the peak. Although regularly-formed, well-defined col- 

 umns are of comparatively rare occurrence, an attempt at such structure 

 may frequently be noticed, and its existence aids greatly the modifica- 

 tion of orographic features. 



Alternating at some places with these hard stratia are other of less 

 durable texture. In certain horizons (speaking of Colorado) flows are 

 found that contain a paste resisting atmospheric influences less success- 

 fully. It is more readily attacked, and instead of the minerals it incloses 

 being the first to decompose, the paste disintegrates and the minerals 

 are contained as such in the detritus, from which often good crystals 

 <jan be obtained. Should a stratum of this description occur on the 

 summit of a mountain or ridge, we will not find the sharp, angular 

 forms of the preceding group, but will observe that they are more 

 irounded, and that the summit itself presents a more or less plateau like 

 appearance. This is due to the erosion and transportation of the softer 

 stratum to such a depth where a harder one may again be reached, and 

 offer more energetic resistance. Within strata belonging to this group, 

 too, we find caves and tunnels. They are formed partly by a widening 

 oi existing fissures, partly by gradual disintegration of loosely cemented 



