588 J. CLIFTON WAEB ON THE GRANITIC, GKANITOID, AND 



discrepancies may also be owing, in part, to variation in the amount 

 of salts held in solution, the rate of expansion of water varying with 

 that amount. It is possible, again, that in very minute cavities the 

 liquid has been appreciably stretched, and thus we may be led to 

 fancy the pressure greater than it really was. To all these points of 

 uncertainty Mr. Sorby has alluded in his paper. 



IV. Summary. 



1. Granites and granitoid rocks may sometimes have been the 

 deep foci of volcanic phenomena, and at other times represent masses 

 formed deeply beneath the surface, consolidated from a state of 

 aqueo-igneous fusion, and widely metamorphosing the overlying 

 strata. 



2. Such deeply formed rocks must have been subject in their 

 formation to great pressures, partly from the weight of overlying 

 rocks, partly from internal pressures acting chiefly from below ; we 

 may call the first downward pressure, and the second surplus or out- 

 ward pressure. 



3. If the outward pressure be relieved by volcanic action, the ivliole 

 pressure to which the consolidating granite in an adjacent part is 

 exposed may be little more than that represented by the depth or 

 downward pressure. 



4. If the outward pressure be not relieved, the total pressure may 

 be very much more than that represented by the depth ; and this 

 unrelieved outward pressure may do work in elevation and contortion 

 of the overlying rocks. 



5. In case 4 the metamorphism effected would probahly be much 

 more widely spread than in case 3. 



6. The microscopic examination of the relative size of the liquid- 

 cavities and their vacuities in the quartz of granitic and granitoid 

 rocks furnishes a probable means of estimating the pressure under 

 which the cavities were filled. 



7. This pressure, being of a twofold character, does not give any 

 idea of the relative values of downward and surplus pressures, unless 

 an approximation to one of these be known. If the depth or downward 

 pressure be known, the surplus represents the outward, or vice versa. 



8. Example. — If the microscopic examination gave a pressure of 

 50,000 feet in rock for a certain granite, and it were known from 

 geological field-observation that in all probability the rock could not 

 have been consolidated at a greater depth than 30,000 feet, there is a 

 surplus of 20,000, which would probably be spent in the work of 

 elevation, contortion, and metamorphism of the overlying rocks. If, 

 on the other hand, the microscopic examination gave a pressure of 

 only 30,000 feet, and geological observation tended to show that the 

 depth of formation was probably 30,000 feet, we might infer that most 

 likely the outw ard pressure was relieved by volcanic phenomena*. 



* In most, if not in all cases, however, of volcanic activity, there would be 

 an excess of unrelieved pressure ; and this would be sufficient to account for the 

 very general upheaval prevailing over volcanic areas, 





