14 PHILOSOPHICAL TRANSACTIONS. [aNNO 1791. 



circumstance lias prevented tlie constituent parts tVom receding completely from 

 each other. Experiments show, that almost all granites melt into a black glass; 

 and perhaps it is no abuse of analogy, nor inconsistent with what has been already 

 remarked, to conclude, that granite, in the state of imperfect fusion, should 

 present a glassy substance, involving the more infusible parts of which this stone 

 consists. 



The Scheibenberg, near Konigsbruck, consists of a stone which Mr. Leske 

 knows not whether to call hornslate, or corneous porphyry. From the descrip- 

 tion it appears plainly to be a whinstone. The colour is dark grey ; it breaks 

 into columnar fragments ; is hard, fine-grained, and sonorous ; little veins of 

 quartz cross it in all directions, and it frequently becomes porphyritic, as in- 

 closing crystals of feldspath. The author himself is afterwards aware of its affinity 

 to basaltes, both in substance and from its assuming the columnar form. In this 

 hill a mass of granite is found imbedded in the whinstone, and on all sides sur- 

 rounded by it, and the mass of granite is in its turn in all directions intersected 

 with veins and stripes of whinstone. Mr. Leske is much struck by this mutual 

 and intimate incorporation ; but he makes no attempt to explain it. In some 

 instances, he thinks an eruption has broke out through the granite ; and in 

 others is at pains to show that these substances are not thoroughly blended, as in 

 the last example, and in that described by Ferber. 



It may be said, and no doubt it sometimes happens, that shivers of granite, 

 broken off by the violence of explosion, arc licked up by melted matter as it 

 moves along ; thus, in volcanic breccias an older lava is inclosed in one more re- 

 cent, and thus what is called primary is sometimes encased in secondary granite. 

 But such an hypothesis is too narrow to embrace all the phenomena. It does not 

 explain the incipient coagulation of the uniform paste into grains, and those the 

 different grains of granite ; nor the diffusion of the constituent parts of granite 

 through the substance of basaltes ; nor the 5th species described by Mr. Ferber. 



In the whinstone rocks of England, which are far more numerous than is com- 

 monly supposed, Dr. B. has often observed in the same hill, l. homogeneous dark 

 grey stone ; 2. fcldspath inclosed in this as in a paste ; and, 3. the paste disap- 

 pearing, and the whole becoming granular, and the grains heterogeneous. Be- 

 sides feldspath, quartz is found in innumerable masses of varying magnitude in 

 many whinstone rocks, and as proper basaltes is but a confused mass of crystals 

 of shoerl, we have all the ingredients of granite ; and why may we not expect to 

 find them incorporated together, and in every state of diffusion and separation ? 



Further, several late observations, from which it has been inferred, that cer- 

 tain extinct volcanos have been seated in the heart of granite, seem to admit of a 

 much more easy explanation, on the su[)position that granite has crystallized from 

 fusion. 1. Volcanic fires reach to a much greater depth than any at which we 



