576 J. CLIFTON WARD ON THE GRANITIC, GRANITOID, AND 



derived from other formulae which it is not necessary here to repro- 

 duce, and taking his observations on the trachyte of Ponza as a 

 basis, we have 



369000 *^=| =p + 4000 feet. 



supposing the temperature of consolidation to have been 360° C. 

 (680° F.). Substituting in this formula the values given in the 

 column of our table headed v, we procure the results given in the 

 second column, which represent in each case the pressure of rock in 

 feet necessary to compress the liquid so that it would fill the cavities 

 at a temperature of 360° C. (680° P.), which is a dull red heat visible 

 in the dark. 



It must be clearly borne in mind, however, that these figures do 

 not represent the depth at which consolidation took place, but only 

 the amount of pressure ; for, as Mr. Sorby has remarked (I.e. p. 491), 

 H in some cases the pressure was probably much greater than that 

 of the superincumbent rocks, for otherwise they could not have been 

 fractured and elevated ; whereas in other cases it may have been 

 much less if the internal pressures had been in any way relieved." 

 This is a consideration of the highest importance in our present line 

 of argument, as we shall presently see. 



1. Skiddaw Granite. — I think it is pretty certain that the Skid- 

 daw Granite was never covered up under a greater thickness of 

 strata than 38,000 feet ; and probably this is above the mark ; 

 whereas, assuming for a moment that the measurement '130 is a 

 correct one to work upon, according to Mr. Sorby's formulae the cal- 

 culated pressure of rock in feet is 52,000, a difference of 14,000 feet. 

 Here, then, the pressure being greater than that of the overlying 

 rocks, it is likely that these last were elevated and fractured at the 

 time, and unlikely that the internal pressures were relieved by 

 volcanic action. If the reverse were the case, the calculated pressure 

 being under the estimated thickness of 38,000 feet, we might gather an 

 argument for the direct connexion of this granitic mass with vol- 

 canic outbursts by which much of the pressure might have been 

 relieved. 



As it is, however, confirmation is rather given to the view that 

 the granite was formed beneath the above estimated thickness of 

 strata and under circumstances of great additional pressure, by means 

 of which that elevation and disturbance of the district was effected 

 which took place at the close of the Upper Silurian period. 



In the consideration of this case, however, it might become a 

 question whether the difference of 14,000 feet between calculated 

 pressure and estimated thickness of overlying beds is not too great — 

 although, when one considers the enormous amount of quiet force 

 required for the gradual elevation and contortion of 38,000 feet of 

 strata, this extra pressure at a temperature of only 360° C. (680° F.) 

 does not seem any too great for the work, in fact it becomes doubtful 

 whether it be enough. 



