336 Williams — Occurrence of Paleotrochis in Mexico. 



In the case of the Paleotrochis the application of the sug- 

 gestion would be in this wise : If a porous rock, had arising 

 through it from below, superheated waters carrying solution 

 of silica, a point might be reached when suddenly the loose- 

 ness of the rock would permit of rapid radiation upwards of 

 heat, while below the temperature of the heat would be high 

 and remain nearly constant for a long period. Such conditions 

 might occur along the plane of a considerable exit of the rising 

 waters, the rocks below being saturated with the heated waters 

 and above the rock being more or less open by the absence of 

 water. I conceive that, at the junction, a condition might 

 occur in which silica would begin to deposit on the interior of 

 a cavity, but the supply of silica, or of silicated waters, would 

 be furnished only as it was abstracted by solidification. Thus 

 at the points where solidification began the cavity would be 

 increased by the crystallization, and into this cavity more sili- 

 cated waters would arise; thus the accretion-surface would 

 increase with the lengthening of the cone by the separation of 

 the apices of the cones ; consolidation taking place at the 

 point of meeting of the solidified silica with the water, and 

 the water reaching that point slowly. 



If some such explanation be the fact, the association of these 

 peculiar forms with gold is explained. Also that such condi- 

 tions should occur in volcanic regions and particularly in the 

 midst of volcanic tuffs, as in the Mexican case, is as would be 

 expected. 



Nov. 22, 1897. 



