LITIIOLOGY. 193 



with brilliancy, and arc then found to have suffered marked physical 

 changes in color, specific gravity, deportment with acids, &;c., to be proof 

 that the granite must have formed at a temperature not above that indi- 

 cated by a dull red. 



Now, it has already been explained that the size of the bubble in a 

 fluidal cavity is dependent on two factors, temperature and pressure, 

 either of which being known the other can be calculated. Assuming 

 the stated temperature as being correct, Mr. Sorby has calculated the 

 pressure in a variety of cases, and finds it, though variable, to be always 

 enormous. Mr. Ward* followed with more extensive calculations, using 

 the same formulae. Comparing the results with those obtained in the 

 field, it was usually found that the pressures deduced from calculation 

 are much larger than those which would result from the pressure of the 

 superincumbent strata that can be proved to have rested upon the rocks 

 investigated. If, for example, the calculations indicate a pressure equal 

 to 50,000 feet of rock, but 30,000 can be found, etc. This indicates that 

 granites are rocks which were formed deep down in the earth, and crys- 

 tallized from a plastic condition, under pressure, which resulted in part 

 from the weight of superincumbent strata, and in part from the lateral 

 pressure which elevated the mountains. Although accuracy can scarcely 

 be claimed for such calculations, their general agreement, when regarded 

 as approximations, renders them valuable. Much more, which is very 

 interesting, has been done to substantiate the accepted theory ; and espe- 

 cially in this connection may be mentioned the experimental researches 

 of Daubree, who has reproduced some of the circumstances in sealed 

 tubes, and crystallized the minerals. Enough has been said, however, 

 to define the method by which the members of our most important 

 group of rocks were formed, and to explain the variety of circumstances 

 in their mode of occurrence which is met with. 



Miiscovite Granite. This is an uncommon rock, and the one specimen 

 that I have to examine comes from Newcastle, where it occurs sparingly. 

 It is therefore a rock of little importance to us. It would seem as if the 

 entire removal of iron from sediments was usually accompanied by the 

 removal of so many other constituents as rarely to leave the material for 



* Quart. Jour. Geol. Sec, Nos. 124 and 125. 

 VOL. IV. 25 



