556 KIRK BRYAN 



The rocks that are affected by this process are hmestone, calcareous 

 sandstone, gypsum, and salt. Silver Spring, in Marion County, 

 Florida, is a well-known example. This spring emerges from 

 several openings in the cavernous Vicksburg limestone into a pool 

 several acres in extent. The volume of flow is about 385,000 

 gallons a minute, or 855 cubic feet a second, a stream sufficiently 

 large to float a steamer.^ 



Lava tubular springs are due to caverns and tunnels in lava 

 flows. These caverns and tunnels are formed through the process 

 of igneous extrusion. When a very liquid lava cools rapidly, a 

 crust of sufficient strength to support itself may form on the surface 

 and the liquid lava below may flow out, leaving an arched tube. 

 Under other conditions the crust may yield to lateral pressure of 

 the underlying liquid lava and form arched tunnels which are made 

 permanent by the solidification of the lava. Principally by these 

 two processes caverns and tunnels may be produced in formations 

 due to successive lava flows, and under favorable circumstances 

 such openings may form channels for the ground water. On the 

 southern flank of the mountain mass which culminates in Lassen 

 Peak, California, large springs break out in Big Meadows, about 

 5 miles from Prattville.^ The water issues from basaltic lava in an 

 area about 100 yards ^in diameter. The flow is 29,000 gallons a 

 minute, or 62 .6 cubic feet a second, and the temperature is 46° F. 

 The springs belong to a series characteristic of the lower slopes of 

 the Lassen Peak mass, and a few of them are shown in Figure 2 

 (p. 533). The low temperature of these springs indicates that the 

 water originates high up on the mountain and flows rather quickly 

 down the slope. It seems probable that the water flows through 

 caverns and tunnels in the lava rather than through the pores of 

 the rock. This interpretation is supported by the existence of a 

 similar series of springs around Mount Shasta. Plutos Cave, north 

 of this mountain, is an example of the kind of caverns through which 

 the water passes. It is in places 60 feet high and 50 feet wide and 



' G. C. Matson and Samuel Sanford, "Geology and Ground Waters of Florida," 

 U.S. Geol. Survey, Wakr-Supply Paper jig (1913), p. 367. 



^ G. A. Waring, "Springs of California," U.S. Geol. Survey, Water-Supply Paper 

 3J^ (1915), P- 330- 



