CLASSIFICATION OF SPRINGS 529 



essential to the formation of the tubes. Certainly the waters of 

 all known geysers carry silica in solution in relatively large amounts/ 



Ebbing and flowing springs occur along the seacoast. During 

 high tide the sea water acts as a dam for the ground water and 

 causes it to flow inland and at higher levels as springs. At low 

 tide the ground-water level falls and the springs are reduced in 

 volume or dry up. On this account the flow of springs adjacent to 

 the shore fluctuates with the tide and may even cease at low tide 

 except close to the shore, where some springs flow only at low tide. 

 Rise and fall of water level due to the same cause have been 

 noticed in wells.^ 



Blowing or breathing springs and wells are characterized by the 

 emission of air, often accompanied by a trumpeting sound. They 

 appear to be due to two causes: (i) rise of the water table, which 

 causes the expulsion of air from cracks and pores of the rock;^ 

 (2) decrease in barometric pressure, which causes a similar move- 

 ment.'' The rise of water brings about blowing at relatively long 

 and irregular intervals, but low air pressure recurs in short cycles. 



Mound and knoll springs occur in arid climates. The water 

 emerges at or near the top of a mound which has been built up by 

 the accumulation of wind-blown sand and dust in the belt of 

 vegetation surrounding the spring. The height of the mound is- 

 Hmited by the height to which the water can rise, for any accumula- 

 tion of sand that is not moist is easily removed because it cannot 

 support the protective vegetation. When the water can rise no 

 higher, the process which builds the mound tends to ceil it over. 

 If the water then finds a new and lower outlet, the mound is 

 drained and subjected to erosion, especially by wind. Mound 



^ F. W. Clarke, "The Data of Geochemistry," 3d ed., U.S. Geol. Survey, Bull. 616 

 (1916), p. 196. 



2 A. C. Veatch, " Fluctuations of the Water Level in Wells, with Special Reference 

 to Long Island, N. Y.," U.S. Geol. Survey, Water-Supply Paper 155 (1906), pp. 10 ff., 

 63-69. H. B. Woodward, The Geology of Water Supply (London, 1910), p. 90. 



3 H. B. Woodward, op. cit., pp. 87-88. 



t A. Strahan, "The Movement of Air in Fissures and the Barometer," Nature, 

 Feb. IS, 1883, p. 375, quoted by Woodward. A. C. Veatch, op. cit.; C. W. Hall, 

 O. E. Meinzer, and M. L. Fuller, "Geology and Underground Waters of Southern 

 Minnesota," U.S. Geol. Survey, Water-Supply Paper 256 (191 1), p. 90. 



