202 . PHYSIOGRAPHY. [chap. 



that, whatever the general state of the earth's interior, there 

 must be at least local masses of molten rock. 



Additional evidence of the existence of heat at great 

 depths is furnished by the temperature of the water yielded 

 by certain springs. Some of the hot springs at Bath, for 

 example, have a temperature of 120° F. Still hotter springs 

 occur in many countries ; and, in volcanic districts, even the 

 boiling-point is occasionally reached. The most remarkable 

 of these hot springs are those known in Iceland as Geysers. 

 Jets of boiling water with clouds of steam are intermittently 

 thrown high into the air with great force and accompanied 

 with loud explosions. The water generally holds silica in 

 solution, as mentioned on p. 124, and this siliceous matter 

 is deposited around the mouth of the hole as an incrustation 

 called sinter. Although the Geysers of Iceland are best 

 known, similar springs are found in New Zealand, and also 

 in the Rocky Mountains of North America. Fig. 58, repre- 

 sents a geyser in the Yellowstone Park, described by Prof. 

 Hayden. No fewer than 10,000 hot springs, geysers, and 

 hot lakes are said to exist within the area of the Yellowstone 

 Park. The geyser, here represented in action, throws jets of 

 hot water to a height of something like 200 feet. 



In some localities, hot water issuing from the ground is 

 mixed with earthy matter ; and streams of thick mud accumu- 

 late round the openings, so as to form conical hills, known 

 as salses, or j/iud volcanoes. Such eruptions of mud, varying 

 considerably in consistency and in temperature, occur, for 

 example, in the Crimea and on the shores of the Caspian 

 Sea. In other cases, hot vapours issue from cracks in the 

 ground, as at the Solfatara, near Naples, where the vapours 

 are charged with sulphur. A large industry has sprung up 

 in the Tuscan Maremma, by utilizing the hot vapours which 

 issue from smoking cracks, known as soffioni, and contain 



