46 VOLCANOES 



The distance to which lava streams extend and the rapidity 

 with which they move are determined by the abundance and 

 fluidity of the lava and the slope over which it flows. Some lavas 

 are so liquid that they flow for many miles, even down moderate 

 slopes, while others are so pasty that they stiffen and set within a 

 short distance of the vent, even on steep grades. Ordinarily the 

 motion soon becomes very slow, though thoroughly melted masses 

 pouring down steep slopes may, for a short time, move very swiftly. 

 One of the lava floods from Mauna Loa moved fifteen miles in 

 two hours, and for shorter distances much higher rates of speed 

 have been observed ; but this is very exceptional. 



The cooling of the surfaces of the lava stream takes place rap- 

 idly, while the interior cools but slowly, and great thicknesses 

 require very long periods of time to become entirely cold. The 

 differences in the rate of cooling produce very strongly marked 

 varieties in the appearance and texture of the resulting rock. The 

 portions which have chilled and solidified very quickly are glassy 

 and form the volcanic glass, obsidian. If the swiftly cooling por- 

 tions have been much disturbed by the bubbles of steam and 

 vapours, they are made light and frothy ; in some cases, as in 

 pumice, they will float upon water. Otherwise, the glass is solid 

 and is usually very dark in colour, resembling an inferior bottle 

 glass in appearance. Microscopic examination shows minute, 

 hair-like bodies in the glass, which are called crystallites, and rep- 

 resent the incipient stages of crystallization. 



Passing inward from the surface of the lava stream, we find 

 the steam bubbles becoming rarer, until they cease altogether, the 

 vapours having escaped while the lava was still so soft that the 

 bubble holes soon collapsed. At the same time the glassy texture 

 of the rock is replaced by a stony character, which the microscope 

 shows to be due to the formation of crystals too minute to be rec- 

 ognized by the unaided eye. Still deeper in the rock the stony 

 texture passes gradually into an obviously crystalline one ; and the 

 slower the cooling, the larger will these crystals be, though in lava 

 streams which have cooled on the surface of the ground, the whole 

 mass, even of the deeper parts, is never coarsely crystalline. 



