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More Isoetes

     Llano and Burnet counties have extensive outcrops of granite, which 
     may be why Isoetes lithophyllum is found there exclusively.  However, 
     limestone is common in Texas, as the area was a shallow sea several 
     hundred million years ago.  Marine fossils are everywhere.
     The type of granite found there is pink, which is due to potassium 
     feldspar.  Pools formed by rainfall would be very soft, while 
     spring-fed pools would be rather hard.
     Rainfall is best described as erratic, with no distinct wet or dry 
     season.  Thus, "vernal" pools can be formed any time of the year, and 
     we have a 300 day growing season.  We have just emerged from a severe 
     drought, and I am sure most pools and springs with Isoetes dried up.  
     Droughts occur about every 5-6 years.