
Sy Fae eo, 
- Parr I. Secor. i.§3.] VOLCANIC CONES. — 245 
_- river-gorges and on the upthrow side of faults, have already (p. 210) 
been noticed. Among the Carboniferous volcanic rocks of central 
Scotland the stumps of ancient tuff-cones, frequently with a central 
core of basalt, or with dykes and veins of that rock, are-of common 
occurrence." 
The materials of a tuff-cone are arranged in more or less re- 
gularly stratified beds. On the outer side they dip down the slopes 
of the cone at the average angle of repose, which may range between 
30° and 40°. From the summit of the crater lip they likewise dip 
inward toward the crater-bottom at similar angles of inclination 
(Fig. 50). | 
3. Mud-cones resemble tuff-cones in form, but are usually 
smaller in size and less steep. They are produced by the hardening 

Fic. 50.—SEcTION oF THE CRATER-RIM OF THE ISLAND OF VOLCANO. 
a, Older tuff; bb, younger ashes; the crater lies to the right. 
of successive outpourings of mud from the orifices already described 
(p. 234). In the region of the Lower Indus, where they are abun- 
dantly distributed over an area of 1000 square miles, some of them 
attain a height of 400 feet, with craters 30 yards across.” 
4. Lava-cones.—Volcanic cones composed entirely of lava are 
comparatively rare, but occur in some younger tertiary and modern 
voleanoes. T'ouqué describes the lava of 1866 at Santorin as having 
formed a dome-shaped elevation, flowing out quietly and rapidly 
without explosions. After several days, however, its emission was 
accompanied with copious discharges of fragmentary materials and 
the formation of several crateriform mouths on the top of the dome. 
Dome-shaped protrusions of trachyte occur in the Auvergne and 
Hifel districts, where their existence has been referred to the more 
infusible and viscid character of their component lavas, and to the 
absence of scoriz and ashes.2 Where, however, the melted rock 
possesses extreme liquidity, and gives rise to little or no fragmentary 
matter it may also build up a low cone as in the remarkable examples 
described by Dana from the Hawaii Islands. On the summit of 
1 Trans. Roy. Soc. Edin. xxix. p. 455. See postea, Book IV. Part VII. 
2 Lyell, Principles, ii. p. 77. 
> See ante, pp. 224, 243, note ; also the remarks upon “ Vulkanische Kuppen,” postea, 
p. 256. 
4 In Wilkes’s Report of U. S. Exploring Expedition, 1838-42. 
