268 
THE TERTIARY VOLCANOES 
BOOK VIII 
grouping is quite irregular . 1 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 .” 2 The deep gorges cut by the 
livers 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 scoriae at the top . 8 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 unmis- 
takably their sources in small depressed cones of very flat profiles. No 
fragmental ejecta (scoriar, lapilli, etc.) have been found in connection with 
these young eruptions.” 1 Such I believe to have been the general 
conditions under which the basalts of the Tertiary plateaux of the British 
Isles were also erupted . 5 
Although we may be convinced, from their general structure and 
illations, 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 lias 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 
1 Captain C. E. Dutton, “ Tertiary History of the Grand Caiion District,” U.S Geol Survey 
(1882), p. 101. ' J 
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. 
3 Tertiary History of the Grand Canon, etc., p. 97). 
4 Nature, xxxi. (1884), p. 49. 
I may again reler 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 ef 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 
s nurture can he traced, it woulfl probably be in vain to look for any single centre of eruption. 
n e 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 ol the earth's crust, would not necessarily destroy the continuity of a comparatively 
tun 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). 
