GROUND-WATER 



215 



of water above. If convection were free and the tube short, the 

 result would be a boiling spring; but if the tube is long, and espe- 

 cially if convection is impeded, the water at some level below the 

 surface may be brought to the boiling-point earlier than at the top. 

 Under these circumstances if even a little water in the lower part 

 of the tube is converted into steam, the steam will raise the column 

 of water above, and it will overflow. The overflow relieves the 

 pressure on all parts of the column of water below the surface. 

 If before the overflow there was any considerable volume of water 



Fig. 174. The cone of Lone Star Geyser, Yellowstone National Park. 



(U. S. Geol. Surv.) 



essentially ready to boil, the relief of pressure following the overflow 

 might allow it to be converted into steam suddenly, and the sudden 

 conversion of any considerable quantity .of water into steam would 

 cause the eruption of all the water above it (Fig. 173). The height 

 to which the water would be thrown depends upon the amount of 

 steam, the size and straightness of the tube, etc. 



It is clear that everything which impedes convection in the 

 geyser tube will hasten the period of eruption, since impeded cir- 

 culation will have the effect of holding the hot water down, and so 

 of bringing the water at some level below the top more quickly to 

 the boiling-point. It follows that anything which chokes the 



