176 DOMAIN X. TKANSILIENr* 



NOME XI. GRANITE AND GRANITIC POR- 

 PHYRY. 



This is also a very common rock. 



The passage from granite to granitic porphyry 

 being one of the most remarkable and important, 

 the following observations of Dolomieu will be 

 found to merit particular attention*. 



" During the great coagulation, to which the 

 primitive mountains owe their construction, it' 

 seems that there have been substances, of which 

 the concurrence, or too great abundance, has 

 impeded or prevented the regular aggregation, 

 in giving the paste a tenacity, in some manner 

 fattening it, to make use of a term applied to 

 mother-waters when they refuse to crystallise* 

 Such are the particles of talc, and of argillaceous 

 and magnesian earths \vhen free. It seems that 

 these earths, naturally unctuous, have prevented 

 the other particles from assuming the places to 

 which the laws of elective aggregation destined 

 them, in causing them to slide on one another. 

 I have pretty generally observed that the super- 

 abundance of magnesian earth chiefly acted upon 



* Journal de Physique, new scries, vol. i. 1794, p. 1Q3. 



