ENDUCH.] ERUPTIVE ROCKS — SPECIAL FEATURES. 101 



are sufficiently numerous, they may readily form in this manner a moun- 

 tain or mountain group of considerable height and extent. Added to 

 the " power of preservation" that these dikes have, with refereuce to the 

 neighboring, more readily disintegrating strata, is the fact that often 

 the formation of the dike-system itself was accompanied by a local ver- 

 tical upheaval, thus increasing the relative heiglit of the beds which 

 afterward owe the position they retain directly to the influence of the 

 intrusive volcanic material. 



Various conditions under which the dike-rock must have cooled, have 

 produced different physical characters. While some of them withstand, 

 most successfully, the atmospheric and other erosive agents, others 

 readily disintegrate by breaking into more or less angular fragments, 

 which are not long sustained at the points of their original occurrence, 

 but, rolling down, form a talus along the base of the dike. The influ- 

 ence of dike-occurrences upon local orographic features is one not to be 

 underestimated. Although apparently unimportant, when seen only in 

 a single representative, they develop the ability of totally changing the 

 face of a region wherever they may be found in sufficient numbers and 

 in the proper arrangement for that purpose. 



SPECIAL FEATURES. 



As special features the various curious results of erosive agents may 

 be briefly alluded to. Among these we may count — 



Caves and tunnels. 



Arches and 



" Monuments. " 



Caves may owe their formation to various causes. Prominent among 

 these are subsidences, excavation by chemico-physical agency, and ero- 

 sion in its widest sense. Only the last-named enters into consideration 

 here. Most frequently such caves are met with in sandstones, volcanic 

 beds, and hard conglomerates. They are due, mainly, to the existence 

 either of strata or locally circumscribed spots that are composed of ma- 

 terial disintegrating more readily than its surroundings. Percolating 

 waters penetrate to any depth that has ever been examined, and should 

 they reach such a stratum or portion of stratum which, for instance, 

 was exposed, the action of the water alone, more particularly, however, 

 that of the frozen water, would produce a gradual, successive scaling 

 oft' of certain parts, until an opening in proportion to the extent of the 

 yielding rock is formed. Adjoining strata will also be attacked, but 

 suffer less than the soft inclosure. 



Arches are, in case they are produced by erosion, the most complete 

 form of caves formed by the same agents. It is natural that they can- 

 not occur unless the conditions are very favorable. It is again a softer 

 stratum or portion of the stratum that is attacked. Instead of having 

 the face of a bluff, however, we must have either an isolated group or 

 narrow wall of the rock. Then the susceptible parts are subjected to 

 erosive agents from both sides. Gradually losing more and more ma- 

 terial, the wall becomes thinner, until finally the aperture is cut en- 

 tirely through, and the arch is completed. Subsequent erosion will 

 give it outlines that are in conformity with the original extent of the 

 soft spot. Another method of formation was observed in an isolated 

 block of sandstone, in the centre of which there was an opening large 

 enough to permit the passage of two men abreast. It was there no- 

 ticed that a vertical narrow fissure traversed the entire height of the 

 huge block. Water accumulating in this fissure had saturated a soft 



