RELA TION OF SOME GRANITES, SANDSTONES <&- CLA YS. 205 



Comparison of the Composition of Granites and Clays. 



Bendorf, it will be seen that the chief differences between them are 

 that the granite contains some soda not found in the clay, and three 

 times as much potash, while the clay contains much more water than 

 the granite. These differences are small compared with those seen in 

 different so-called granites, though they would determine distinct 

 felspar formation. The percentage of lime and magnesia in clays 

 must necessarily vary with the organisms clays contain, so that the 

 only difficulty which presents itself exists in the small percentage of 

 soda and the large percentage of water. Some alum-shales in Sweden 

 contain 3 to 8 per cent, of potash, and some are free from water. 

 From the fact that crystals of sodium chloride are met with in purbeck, 

 triassic, and carboniferous clays in this country, the deficiency of soda 

 is not perhaps insuperable, even without appealing to probable sources 

 of salt supply, in the contact of sea-water with heated regions beneath 

 the earth's surface, or growth and decay of marine plants. And the 

 presence of combined water in clay, though greater than the quantity 

 which is found in granite, is perhaps not more than we may assume 

 Avould be easily expelled or altered by chemical combination, under 

 the conditions of igneous fusion, since it often disappears in slates. 

 If, then, it is no rare circumstance for certain sandstones and clays 

 to have an average chemical composition which is nearly identical 

 with that of granite, we may fairly believe that such strata, 1 under 

 requisite conditions of heat, pressure, and cooling, such as would be 

 presented in the central axis of a mountain mass, would become 

 changed by metamorphic action into granite rock. 2 



1 See pp. 206 and 210. 



- It may not here be out of place to draw attention to the celebrated instance 

 at Carlingford, in which Dr. Haughton affirms that veins of granite penetrating 

 into carboniferous limestone become converted into syenite, by assimilating a 

 certain portion of the limestone. Mr. R. H. Scott confirms Dr. Haughton's de- 

 termination, so that the facts may be presumed to be clear ; yet we cannot help 

 drawing attention to the circumstance that the Carlingford granites all contain 

 7 to 8 per cent, of alkali, and Dr. Haughton's analysis gives no trace of alkali in 

 his syenitic rock. Such a result would not have been anticipated from adding 

 limestone t > granite substance under such conditions. 



