THE GEYSER TUBE-SHAFTS. 67 



The whole excess of heat above 100° C. must of ne- 

 cessity be employed in generating steam. The water 

 therefore displaced and carried up by the steam given 

 off at the same time at all points, and mingled with 

 it into w r hite foam, gushes up in a constant 

 stream, roaring and hissing from the welTs mouth. 

 — This is exactly what may be observed on suddenly 

 opening a valve at the top of a steam-boiler almost 

 filled with water heated far above the boiling-point. 



The formation, however, of flint-sinter goes on 

 around these springs unceasingly from year to year. 

 The spring-shaft continues to grow and the ground 

 about it to rise ; and the flow of the spring must 

 at last cease, when, perhaps after the lapse of 

 centuries, the ever-increasing weight of the water- 

 column has compelled the liquid to find another 

 escape, or the supply from below may have been 

 diminished, so that in consequence of the cooler 

 water from above continually sinking down, no 

 part of the column of water can any longer reach 

 the boiling-point. Thus great water basins are 

 formed, moulded in solid tufa, and filled with water 

 which is as clear as crystal, and of a splendid 

 greenish-blue, and which either is quite still or 

 runs off very slowly. There are several of these 

 basins near the Great Geyser, and at many other 

 places in Iceland ; and in their depths may still be 

 seen the mouths of the ancient spring-shafts. 



The grand intermitting springs of Iceland, the 



f 2 



