These characteristic features of one distinctive type of volcanic action have been repeated over a vast region, or rather a whole series of regions, in Western America, the united area of which must equal that of a considerable part of Europe. From Idaho, the basalt-fields may be followed southwards interruptedly into Utah and Nevada, and across the great plateau-country of the cañons into Arizona and New Mexico, northwards into Montana, and westwards into Oregon. The tract which has as yet been most carefully traversed and described is probably that of the high plateaux of Utah and Arizona. Thus on the Uinkaret plateau, which measures some 45 to 50 miles in length by 8 to 12 in breadth, a thick covering of basalt has been spread composed of many successive flows. Between 160 and 170 separate cones have been counted on this area, most of them quite small, mere low mounds of scoriæ, though a few reach a height of 700 or 800 feet, with a diameter of a mile. From three to seven or eight may be found in a row, as if springing from a single line of fissure. But generally the grouping is quite irregular.[290] My friend Captain C. E. Dutton, from whose admirable memoir these details are quoted, remarks further that among the Utah plateaux no trace of a cone is to be found at or near some of the most recent basalt-fields, and that the most extensive outpours are most frequently without cones. "The lavas," he adds, "appear to have reached the surface and overflowed like water from a spring, spreading out immediately and deluging a broad surface around the orifice."[291] The deep gorges cut by the rivers through these thick accumulations of horizontal or nearly horizontal basalts, have here and there revealed parallel dykes that traverse the rocks, and in at least one case have shown the dyke running for half a mile up a cliff and actually communicating with a crater of scoriæ at the top.[292] Again, in New Mexico, Captain Dutton noticed vast tracts of younger basalt, about which "a striking fact is the entire absence of all distinguishable traces of the vents from which they came. Some of them, however, indicate unmistakably their sources in small depressed cones of very flat profiles. No fragmental ejecta (scoriæ, lapilli, etc.) have been found in connection with these young eruptions."[293] Such I believe to have been the general conditions under which the basalts of the Tertiary plateaux of the British Isles were also erupted.[294]
[290] Captain C. E. Dutton, "Tertiary History of the Grand Cañon District," U.S. Geol. Survey (1882), p. 104.
[291] Captain C. E. Dutton, "Geology of the High Plateaux of Utah," U.S. Geol. Survey of the Rocky Mountain Region (1880), pp. 198, 200. See also pp. 232, 234, 276 of the same Monograph for additional examples.
[292] Tertiary History of the Grand Cañon, etc., p. 95.
[293] Nature, xxxi. (1884), p. 49.
[294] I may again refer to Hopkins's Researches in Physical Geology, where the conditions of the problem here discussed have been distinctly realized. Speaking of the ejection of lava from a number of fissures, he remarks that the imperfect fluidity of the melted material "would seem to require a number of points or lines of ejection as a necessary condition." "If there were only a single centre of eruption, a bed of such matter approximating to uniformity of thickness, could only be produced on a surface of a conical form." "Where no such tendency to this conical structure can be traced, it would probably be in vain to look for any single centre of eruption. On the supposition, too, of ejection through continued fissures, or from a number of points, that minor unevenness of surface which must probably have existed under all circumstances during the formation of the earth's crust, would not necessarily destroy the continuity of a comparatively thin extensive bed of the ejected matter, in the same degree in which it would inevitably produce that effect in the case of central ejection" (Cambridge Phil. Trans. vi. 1835, p. 71).
Although we may be convinced, from their general structure and relations, that the stratified lavas of these plateaux have been poured out from fissures and not from great central cones, it must obviously be difficult to obtain demonstrative evidence of this origin from any single section. Of the thousands of dykes which traverse the British plateaux and the ground around them, I am not aware of a single one which can be actually seen to have ever communicated with the surface. The very process of denudation which has revealed these dykes has at the same time removed all trace of any former connection they may have had with the surface. The only places where we may hopefully search for the missing evidence are the fronts of the escarpments. On these precipices dykes may sometimes be seen to end off at some particular platform among the basalt-sheets, but I have never found a case which could be confidently cited as an example of lava rising in a fissure and spreading out as a superficial sheet. That this connection may eventually be found when a more detailed survey is made of these great sea-walls I fully anticipate.
In recently mapping the basalt-plateau of Strathaird in Skye, Mr. Harker has made some interesting observations regarding the probable connection of the dykes with the plateau basalts. He has noticed that the flanks of Slat Bheinn, a portion of the plateau, are abundantly traversed by dykes containing numerous enclosed pieces of gabbro, while the basalt on the summit of the plateau is full of similar fragments—an occurrence not observed elsewhere. It is conceivable that the gabbro-bearing basalt-sheets are sills, but Mr. Harker has found no proof that they are so, the evidence so far as it has been collected being rather in favour of the view that these sheets are superficial lavas, and that they have been supplied from the dyke-fissures.
Various considerations suffice to assure us that actual instances of the outflow of the basalt from its parent fissures should be expected to be exceptional. The absence or scarcity of beds of scoriæ among the basalt-plateaux may be taken as an indication that the lava as a rule flowed out without the formation of cinder-cones, and therefore that these conspicuous monuments of the eruptive vents were probably always rare in Britain. If the lava was poured out tranquilly from one or two points along a fissure which were subsequently buried under floods of similar lava issuing from other fissures, the chances that such points of emission should be laid open along the front of any escarpment are small. And, even when so exposed, it might be difficult to feel sure that the dyke below was really the feeder of the basalt above, unless the cliff were accessible and the rocks could be scrutinized foot by foot. These elements of uncertainty are happily removed where the volcanic energy has drilled well-marked funnels of discharge and left them filled with the erupted materials, as will be narrated in the next chapter.