ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 357 



strata. His remarks on this point are as follows : " Let us imagine 

 that an igneous mass crystallizing as it slowly cools, is confined 

 between two parallel planes, which exert botli pressure and re- 

 sistance; the cooling, and consequently the solidification will com- 

 mence at and proceed from these enclosing planes. Now, if in 

 the solidifying mass the conditions exist for the development of 

 many lamellar bodies (such as crystals of mica) then each of 

 those bodies will, in consequence of the pressure, assume a position 

 parallel with the enclosing surface, and the rock will be furnished 

 with a plane parallel structure more or less distinct. If, further, 

 the process of solidification does not progress regularly, but with 

 periodic interruptions, then the rouk would be divided into layers 

 lying parallel to the enclosing planes. If the whole mass, during 

 the progress of the solidification was in regular motion up and down, 

 then there would be developed in each a linear parallel structure 

 or distension of the rock more or less distinct." * Whether the 

 parallel structure of the gneiss of the Primitive formation may be 

 attributed to causes similar to those here indicated, is a question 

 reserved for consideration in the third part of this paper. Mean- 

 while it may be remarked that the granite occurring in beds in 

 that formation, between the zones or layers of gneiss is so intimately 

 connected with the latter rock by lithological transitions that it 

 would seem to be altogether inseparable from it, and that the 

 same origin attributed to the one must belong to the other. In 

 the gneiss-granite of the mountains of Lower Silesia, the granular 

 and slaty modifications of that rock are, according to Von Raumer, 

 regularly interstratilied with each other. In Podolia, according 

 to Von Blode, granite and gneiss together form a whole, to which 

 a contemporaneous and similar mode of formation is ascribable. 

 In Scandinavia and Finland, in the central plateau of France, in 

 Scotland, in Brazil, and Hungary the same relations betwixt granite 

 and gneiss exist. " En Hongrie,"f says Beudant, "ces deux roches 

 se montrent toujours ensemble et uniquement ensemble, elles ne 

 forment pas des couches alternatives, mais une seule et nieme 

 masse." If, therefore, granite, as we have seen, is undoubtedly 

 igneous, then the primary gneiss must be of the same origin, and 

 in this manner we obtain a proof of the original state of the 

 igneous fluidity of our globe. Gneiss is the oldest formation, and 

 if it can be reasonably shown to be igneous, then it must have 

 been the rock first solidified ; and previous to this, it, as well as all 



* Lehrbuch, I, 496. 



f Voyage en Hongrie, III, p. 19. 



