152 



GNEISS. 



Original deposition from water; a supposition con- 

 firmed by its common prolonged and parallel direction 

 with the other strata. It has not yet indeed presented 

 any decided marks of that mechanical arrangement 

 which so often occurs in other stratified rocks ; since I 

 must explain the parallelism of the mica, which has 

 been supposed a proof of such arrangement, in a very 

 different way. In hypersthene rock, an unstratified 

 member of the trap family, the crystals of that mineral 

 often occur in a similar laminar manner, so as to com- 

 municate a fissile tendency to it. And in Kerrera, mica 

 itself is thus found, not only in a mass of trap, but in 

 a vein of the same substance, with the same parallelism 

 to the sides of the vein as it has to the plane of the 

 stratum in micaceous schist; while a similar disposi- 

 tion of mica and pinite both, is common in veins of por- 

 phyry. This cannot have resulted from mechanical 

 subsidence; it must be considered as the result of that 

 common polarity by which the distinct crystals in a 

 common mass are so often regulated. And it has al- 

 ready appeared,, that as granite is the produce of fu- 

 sion, the same law may be extended to the granitic 

 gneiss ; while the form of stratification has been pre- 

 served under those circumstances, as in the stratified 

 traps, and in the siliceous schists by which they are oc- 

 casionally accompanied. Diminishing influences of 

 heat will account for the several other varieties of this 

 rock, as they do for the different primary strata : while 

 it is almost a repetition to suggest why such actions 

 should have induced the chemical changes found in 

 some of the most distinctly stratified varieties, why 

 greater chemical changes should be accompanied by 

 a more obscure stratification, ending in the final pas- 

 sage to granite, why the proximity of this rock is so 

 generally accompanied by gradations between the 

 two, and why, wherever granite veins abound, the in- 



