HEAT. 



275 



increase as the temperature decreased. Copious streams would have the 

 smaller angle, while small streams would give increased pitch, and drops 

 might make a vertical column. The facts with regard to lavas are the same 

 in principle ; for basaltic cones, as in those of driblet origin, may have high 

 angles, even 90°. 



238. 



Gothic Mountain, Colorado. A trachytic mass overlying Cretaceous rocks. 



If lavas were as liquid as water, cones of sensible slope would be 

 impossible ; the most liquid have sufficient viscidity or cohesion to cause 

 some resistance to free movement, and the slope is, in a sense, a measure of 

 this resistance. 



2. Dependence of the forms of lava-cones on place and amount of discharge. 

 — Since a cone diminishes in diameter upward, a flow of lava from the summit 

 region, having like width throughout, would cover a much larger part of the 

 circumference in the upper part than in the lower. The part of the cone 

 below would require in fact a great number of ordinary lava streams to make 

 one coat over the surface. The consequence of this condition is that such 

 discharges add to the height and make the cone steeper above, and give it 

 also a concave outline. But if the flows commence for the most part a little 

 below the summit, from an eighth to a sixth of the height, the upper part 

 will be widened and the cone take the form of a low dome, like Mount Loa ; 

 or if the streams come from fissures in the lower part of the cone and spread 

 beyond the base, the cone will be flattened below, and the lower part of the 

 profile will be made concave. 



Lava-cones often, perhaps generally, derive an oblong or parabolic form of area from 

 their origin over a fissure. The fissure was made by a profound rupture of the earth's 

 crust, and probably the location of the crater was fixed by its intersection with a trans- 

 verse fissure ; but along the larger of the fissures an elongate form is given to the crater. 

 The chief focus of action is usually toward one extremity. Over the slopes of the moun- 

 tain, the belt in the direction of the longer axis is likely to be the region of most frequent 

 eruptions and of long lines of steaming fissures. The craters of both Mount Loa and 



