32 GROVE KARL GILBERT— DAVIS IMEMOIES [vo A i™xxt 



The considerable duration of the basaltic period-:— the latest phase of volcanic activity on 

 the plateaus— and the recency of its last eruptions are shown by the contrast between the 

 erosional isolation of several basalt-capped tables and the freshness of certain neighboring 

 cones and flows. Thus the extensive flows that proceed from the many cones named the 

 Marcou Buttes, in western New Mexico, "follow the present surface, and are almost incomparably 

 newer than the Acoma and Mt. Taylor plateaus, the clift borders of which surmount the 

 recent flows by 500 or 1000 feet." Powell is quoted — and this is of interest as showing the 

 free interchange of results between the two explorers before either of them had published his 

 report — regarding the lava cap of Uinkaret Mountain, the body of which is a "Triassic island" 

 or remnant of the Carboniferous "terrace" north of the Colorado Canyon; and from this 

 Gilbert draws the inference that, since the capping flow was erupted, the Vermilion Cliffs of 

 Triassic sandstone, 1,500 feet in height, have retreated 30 miles; he adds that younger cones 

 "dot the intervening plain" (136). The youngest eruption known to Gilbert is found in a 

 group of cones on the Sevier Desert in the basin range region, of which he gives a detailed 

 account. Some of them — 



may fairly be called modern, although there is no tradition of their eruption . . . Only the consideration of 

 the extreme aridity of the climate can countenance the possibility that centuries, instead of years merely, may 

 have elapsed since the termination of this eruption . . . Indeed, when we compare the stupendous denudation 

 that has transpired during the period of basaltic vulcanicity in this region, with the differential film that has 

 been removed since this last manifestation, and when we consider, in addition, that intermittence is a char- 

 acteristic of volcanic activity, we are not merely permitted to think of a renewal of that activity as possible, 

 but logically compelled to regard it as probable (136). 



He elsewhere wrote, regarding the recent cones in the Sevier Desert: 



In passing, it may be noted, for the benefit of those who base theories on the littoral distribution of vol- 

 canoes, that this locality is six hundred miles from the Pacific ocean. 4 



More pertinent to Gilbert's other work is a conclusion concerning the dynamics of vol- 

 canic eruptions, which is stated, after an account of some of the larger volcanic structures, as 

 follows : 



It is well worthy of note that the majority of these eruptions among the Plateaus rest upon nearly level 

 strata, and that where they are associated with inclined strata, such inclination is seen to pertain to a 

 structure extending far beyond the volcanic outburst, and evidently not dependent on it as a cause . . . 

 This remark applies not merely to the eruptions of basalt, which we know from the narrowness of its dikes and 

 the easy slope of its currents to have been usually a tolerably thin fluid, but also to the most viscous trachyte, 

 which, in the case of San Francisco mountain, for example, has been built, not a scoriaceous mass, but a 

 pyramid of compact lava, to a height of nearly 5,000 feet, with slopes of 10° and 20°. It is by no means 

 impossible, it is probable, rather, that in upheaved ranges, uprising lavas sometimes force apart rock masses, 

 already greatly dislocated so as to open the broad fissures, in which their dikes are occasionally found. But the 

 idea that the ridges of corrugation are lifted by the eruptive rocks that are associated with them — an idea that 

 finds frequent expression in the phrases "upheaved by trap," "upheaved by granite "—appears deserving to 

 be laid on the shelf along with the cognate idea of "craters of upheaval" (130, 131). 



In spite of this well-grounded protest, the idea of "volcanic upheavals" is still strongly 

 rooted in the popular mind, as if it were an established geological principle. On the other hand, 

 not only did Gilbert's later work on the Henry Mountains give much countenance to the pos- 

 sibility of surface upheaval by underground intrusions, but in another part of his Wheeler 

 report the deep-seated forces which caused large eruptions in the province of the basin ranges 

 are held to be identical in their subterranean loci and in their action with the deep-seated 

 vertical forces which caused the upheaval of the range fault blocks, as will be pointed out 

 more fully on a later page. 



DIASTROPHISM : FRACTURES AND FLEXURES 



The displacements of the basin range province will be discussed in a later section. As to 

 the neighboring plateau province, Gilbert, somewhat later than Powell, discovered that some 

 of the great north-south blocks into which the province is divided are separated by rather 



< Proc. Amer. Assoc. Adv. Sci. for 1874, 1876, Pt. II, 30. 



