2o8 NATURAL SCIENCE. mav. 



of the Glacial Epoch and the still later alluvial cones of the 

 torrential streams. 



The uplift of the plateau-districts, as already stated, has been 

 differential. The high plateaux bordering the Grand Canon stand at 

 different levels. They have horizontal or very gently inclined 

 surfaces, and are separated by cliffs often 2,000 feet in height. 

 Examination has shown that one geological horizon, the summit of 

 the Carboniferous, forms the surface of all these plateaux ; conse- 

 quently the cliffs are actual " cliffs of displacement." In most cases 

 they are fault-scarps, only slightly modified. Sometimes a mono- 

 clinal fold is found instead of a fault, the two not being essentially 

 different, but passing into one another. The structure of the country 

 is, then, that of a number of blocks uplifted independently and to 

 different elevations, and bounded by faults or monoclinal fiexures. 

 The mountain chains of the region have in general the same kind of 

 structure with more complication and more modified by subsequent 

 erosion. This type of mountain-structure is known to American 

 students of orography as that of the Great Basin ranges. It is charac- 

 terised by broad monoclinal folds, normal faults, and a form due 

 in the first place to the mountain-building forces. It is strongly con- 

 trasted with the Appalachian type with its closely compressed acute 

 folds and reversed faults and a contour due entirely to the agents of 

 erosion. The one type speaks of vertical elevation, the other of 

 lateral compression. 



The features of interest to the physical geologist exhibited in the 

 region are too many to be noticed in a short article. In some districts 

 igneous rocks have played an important part in the structure of the 

 country. Besides the typical laccolites of the Henry Mountains, so 

 lucidly described by Gilbert, there are great bodies of intrusive rock 

 connected with much more extensive uplifts. Extirict volcanoes and 

 lava-flows of various ages are seen in large numbers on some of the 

 high plateaux. The lavas are harder than the sandstones and other 

 strata of the district, and so the older flows often form the capping 

 of broad, flat-topped " mesas " standing high above the surface of the 

 surrounding country which had no such protection. Later outbursts 

 of lava on the margin of the elevated tract may extend the protected 

 area, and from successive periods of eruption and erosion arise large 

 mountain-masses consisting externally of lava but internally of 

 stratified rocks long ago removed from the surrounding plateau. 

 Such is the character of the Uinkaret Mountains and of Mount San 

 Francisco, the latter rising nearly 6,000 feet above the plateau on 

 which it stands. 



Most of the features noticed above, and many others of equal 

 interest, are typically displayed in the Grand Cafion of the Colorado, 

 an unrivalled example of erosion in an arid climate and in a rising 

 region. Near its upper end it has an average width of ten or 

 twelve miles, with a depth of more than 6,000 feet. Between the 



