
i a 
» = Rr. ee ee * os 2 a * Y= —s 
= Fa ae - “s et? 
5 4 ) Sens Saab} 
256 DYNAMICAL GEOLOGY. —— [Boox II. 
and other regions, ought not rather to be regarded as the remaining — 
99 
roots of true volcanic cones, like the “necks” so abundant in the ~ 
ancient volcanic districts of Britain (Book IV. Part VII). If the tuff 
of a cone up the funnel of which lava rose and solidified were swept 
away, we should find a central lava plug or core resembling the 
volcanic “heads” (vulkanische Kuppen) of Germany. Unquestion- 
ably, lava has in innumerable instances risen in this way within 
cones of tuff or cinders, partially filling them without flowing out 
into the surrounding country." 
But while, on either explanation of their origin, these volcanic 
“heads” find their analogues in the emissions of lava in modern 
volcanoes, there are numerous cases in old volcanic areas where the 
eruptions, so far as can now be judged, were not attended with the 
production of any dome, cone or crater. In former geological ages, 
and perhaps even in the existing period, extensive eruptions of lava 
without the accompaniment of scoriz or with hardly any fragmentary 
materials, have taken place over wide areas from scattered vents, but 
more usually it would seem from lines or systems of fissures. Vast 
sheets of lava have in this manner been poured out to a depth of — 
many hundred feet, completely burying the previous surface of the 
land and forming wide plains or plateaux. These truly “massive | 
eruptions ” have been held by Richthofen? and others to represent: 
the grand fundamental character of vulcanism, modern volcanic 
cones being regarded merely as parasitic excrescences on the sub- 
terranean lavya-reservoirs, very much in the relation of minor cinder 
cones to their parent volcano.* 
Though a description of these old fissure- or massive-eruptions 
ought properly to be included in Book IV., the subject is so closely 
connected with the dynamics of existing active volcanoes that an 
account of the subject may be given here. Some of the most re- 
markable examples of this type of voleanic structure occur in western 
North America. Among these that of the Snake River plain in Idaho 
may be briefly described (Fig. 63). Surrounded on the north and east 
by lofty mountains, it stretches westward as an apparently boundless 
desert of sand and bare sheets of black basalt. A few streams 
descending into the plain from the hills are soon swallowed up 
and lost. The Snake River, however, flows across it and has cut out 
of its lava-beds a series of picturesque gorges and rapids. The — 
extent of country which has been flooded with basalt in this 
and adjoining regions of Oregon and Washington has not yet been 
accurately surveyed, but has been estimated to cover a larger area 
than F'rance and Great Britain combined. Looked at from any 
point on its surface, one of these lava-plains appears as a vast 
level suiface like that of a lake-bottom. This uniformity has been 
1 Von Seebach, Z. Deutsch. Geol. Ges. xviii. p. 643. F. von Hochstetter, Neues Jahrb. 
1871, p. 469. Reyer, Jahrb. K. K. Geol. Reichsanstalt, 1878, p. 81; 1879, p. 463. 
2 Trans. Acad. Sei. California, 1868. 
% Proc. Roy. Phys. Soc. Edin. y. 236. Nature, xxiii. p. 3. 
