280 THE CANADIAN NATURALIST, [Vol. vi, 



the theory of the original igneous fluidity of the globe be well 

 founded. The enormous degree of heat, which only could have 

 occasioned such a condition, could not have disappeared suddenly, 

 A gradual decrease of temperature must have taken place from 

 the time when the solidification of the earth be2;an down to 

 recent geological periods. It follows that this gradually decreas- 

 ing temperature must have had more or less influence upon the 

 cooling of the various rocks protruded through the earth's crust 

 during different geological ages. Those which appeared in earlier 

 periods must have cooled when the earth's temperature was very 

 high, and must therefore have enjoyed the most favorable con- 

 ditions for slow and perfect crystallization and great contraction 

 of volume, while on the other hand, those which were erupted in 

 later ages must have appeared at a time when the temperature 

 had much diminished, and consequently they must have solidified 

 much more rapidly, crystallised much more imperfectly, and ex- 

 perienced less increase of density than their predecessors. Thus 

 there can be distinctly traced a very decided connection between 

 the universally accepted theory of the earth's original fluid con, 

 dition and many of the facts which have been here stated with 

 regard to the density of original rocks. 



But although, generally, definite relations can be shewn to 

 exist between the age and texture of rocks, it is not to be sup- 

 posed that this is invariably the case, that there are no exceptions 

 to the rule. It is not to be forgotten that other conditions be- 

 sides the temperature of the earth's surface may have exerted 

 their influence. Thus it is frequently the case that veins or 

 dykes of diorite have in the centre a distinctly compound tex- 

 ture, while toward the sides they become almost impalpable. 

 Then again beds of basaltite are often seen to be in the upper 

 part and at the bottom fine-grained and compact, while in the 

 middle they are small-grained and variolitic in texture. It is 

 also frequently to be observed that masses of granite distinctly 

 granular in the centre, assume towards the periphery a schistose 

 texture, the direction of which is most generally parallel to the 

 line of junction with the neighbouring rock. Thus it appears 

 that in the solidification of a rock, the space which it occupied, 

 the pressure to which it was exposed, the temperature of the 

 enclosing rocks at the time of eruption, and the circumstances 

 under which it was erupted, whether, for instance, on land or 

 under water, must have influenced more or less its resulting 

 density as well as its texture. 



