113 ANNALS NEW YORK ACADEMY OF SCIENCES 



sense. It is believed that the openings in the rock were in general only 

 of capillary dimensions. Great obstacles were thus presented to the easy 

 circulation of water in the lower part of the channels and to the free 

 escape of steam in the upper part. The temperature of vaporization was 

 thus increased and superheating permitted. The maximum temperature 

 possible imder the circumstances was that corresponding to the pressure 

 to which the water was subjected due to the hydrostatic head. Before 

 the temperature fell to this point, the water was probably converted into 

 steam and escaped in this form, but it is not believed that any important 

 recrystallization was effected, until the temperature reached the point at 

 which the liquid form was retained. 



On account of uncertainties as to the thickness of the lava sheet, no 

 close estimate can be made as to the probable degree of superheating, but 

 some approximation can be arrived at. If a thickness of 300 feet is 

 assumed as a probable maximum, the pressure at the bottom of a column 

 of water of this height would be 130 pounds per square inch, or, taking 

 account of additional atmospheric pressure, 145 pounds per square inch. 

 The corresponding temperature of vaporization is 180° C, and this may 

 be considered as a probable maximum for the temperature which the 

 solutions attained. jS^o great degree of accuracy is to be claimed for such 

 an estimate, but it will serve to show the probable order of superheating, 

 and it indicates that no approach to the critical temperature of water 

 (364.3° C.) was attained. Nevertheless, the minerals formed at this stage 

 were similar to those which are found in highly metamorphosed schists 

 and gneisses. 



From the maximum, the temperature gradual^ fell as the mass of rock 

 cooled. There are indications that, to the last, the temperature was some- 

 what above average climatic temperature. Among the products in the 

 last stages, actinolite and chalcopyrite appear to have been formed, and 

 these would hardly be expected as products of cold solutions. 



The general process was that of the slow but constant and uninter- 

 rupted cooling of a mass of igneous rock through which aqueous solutions 

 were percolating, deriving their temperature from the inclosing rock and 

 cooling as it cooled. Material for solution was contributed by the basalt 

 and by the previously evolved sublimates, and reactions followed by which 

 new minerals were crystallized out. With progressive fall of temperature, 

 conditions of chemical equilibrium within the solutions were shifted, and 

 new species were deposited. In these later changes, the material which 

 participated was derived both from the first-deposited minerals, which 

 had now become unstable, and from new supplies of igneous rock to 

 which access was gained. 



