88 THE WORLD'S LUMBER J?OOM. 



limestone, potash, soda, iron, magnesia, manganese, and 

 various compounds of silica ; and then, when the water is 

 impregnated with these, especially with the salts of potash, 

 soda, lime, and magnesia, its dissolving power is again 

 greatly increased, and far surpasses that of water which is 

 merely charged with carbonic acid. 



It is no wonder, therefore, that hot springs contain a large 

 proportion of mineral matter, and contribute much to the 

 wear and tear of the rocks through which they pass. 



At Terceira the hard lava rock from which the hot springs 

 issue first becomes earthy and covered with red speckles 

 from the rusting of the iron which it contains; then it 

 gradually turns soft, and at length even the glassy crystals of 

 felspar dissolve, and the rock is converted into clay, some of 

 which is quite white from the removal of the iron, and looks 

 like the finest prepared chalk, while some is bright red from 

 the accumulation of iron oxide or rust. 



In England we are far removed from any active volcano, 

 yet we have at Bath hot springs, whose heat, though not so 

 great as those of Aix-la-Chapelle and other places, is still 

 rather startling when we come to consider what it means. 

 The Bath water is not remarkable for the amount of 

 mineral matter which it contains, yet Professor Ramsay has 

 calculated that if solidified, it would in one year form a 

 pillar 140 feet high and nine feet in diameter. All this is 

 now carried away unseen to the Avon, and by the Avon to 

 the sea. 



The most wonderful hot springs yet known are those 

 called the Mammoth springs of the National Park of Yellow- 

 stone, a tract of country in the heart of the Rocky Moun- 



