ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 401 



centagedown to 8 p.c. If we continue the analogy, and suppose 

 the properties of these slags, to correspond somewhat to those of 

 the eruptive rocks having a similar chemical composition, we 

 may find a clue to the explanation of the various forms of 

 deposition, and other characteristics of the latter. Thus it 

 is well known that the slags in which silica preponderates flow- 

 sluggishly and solidify slowly, while basic slags flow quick and 

 hot and harden suddenly. It may reasonably be concluded, that 

 the rocks of igneous origin would act similarly, and that conse- 

 quently granites, porphyries and trachytes would be more viscid, 

 and have better time for cooling and crystallizing, than the more* 

 basic greenstones, melaphyres and basalts. The greater frequency 

 of impalpable and finely granular varieties among the latter 

 rocks would be in this way accounted for. 



We now proceed to consider what must have been the conse- 

 quence of the gradual radiation of heat from the igneous globe. 

 " I know of no mode," says McCullooh,* " in which the surface 

 of a fluid globe could be consolidated but by radiation, while of 

 the necessity of such a process I need not again speak. The 

 immediate result of this must have been the formation of rocks 

 on that surface; and if the interior fluid does now produce the 

 several unstratified rocks, the first that were formed must have 

 resembled some of these, if not all. We may not unsafely infer 

 that they were granitic, perceiving that substances of this charac- 

 ter have been produced wherever the cooling appears to have 

 been most gradual. The first apparently solid globe was there- 

 fore a globe of granite, or of those rocks which bear the nearest 

 crystalline analogies to it." To these utterances we must in the 

 main assent, inquiring however whether the relations existing at 

 the time of this first solidification might not have given rise to 

 the formation of schistose granite or gneiss. Nothing is more 

 conclusively established, than that there exists, at the present day 

 in the atmosphere and ocean, a series of currents, caused by or 

 attributable to the diurnal motion of the earth. May not similar 

 currents have been in operation in the fluid igneous material, 

 during the first solidification of the earth's crust ? Is it not pos- 

 sible that after this solidification had commenced, the outer shell 

 may have moved quicker than the fluid interior, from east to 

 west, and that the gradual accumulation of crystallized rock on the 

 intqpior of the crust may have taken place under circumstances- 



* System of Geology, vol. ii. p. 417. 



