18 THE ORIGIN AND ALTERATION OF ROCKS. 



interior have been liquid since the Azoic time, it may be replied that if 

 contraction suffices to keep up the heat of the sun to an approximate 

 uniformity, so too the contraction of the earth would tend to maintain a 

 uniform temperature in the earth's interior ; a point that it is necessary to 

 consider in all discussions relating to the earth's age. It may again be 

 suo-o-ested that, while basic rocks of the sauie character as those seen to- 

 day were erupted in the early ages of the earth, yet there has been on the 

 whole a progression from the acidic to the basic, in relative abundance, from 

 earlier to later times. Furthermore, owiug to the irregularity in thickness 

 with which the earth's crust has apparently solidified, great diversities would 

 be expected to exist in that part immediately below the crust in different 

 portions of the earth,* 



Whether volcanic and all other eruptive rocks came from material that 

 has never cooled to a solid state since the earth began to solidify, or whether 

 they are derived from a portion that solidified, but has since been reliquefied, 

 is a problem for the future, the solution of which hinges on the origin of 

 the partially destroyed materials in the rocks themselves, — were they 

 caught up on tlie passage of the lava to the earth's surface, or are they 

 the remains of a prior crystallization ? 



If we turn to Sorby's method for determining the origin of rocks by the 

 inclusions in the contained minerals, we find that it may possibly answer 

 in recent, surface-formed rocks; but that in the old and altered forms it 

 seems to carry us astray, and serves but to retard tlie advance of our knowl- 

 edge of rock formation. This is especially the case if the secondary min- 

 erals, like quartz, have been formed later in the rock in question by the 

 action of hot waters. Conclusions regarding the origin of a secondary or 

 forei2:n mineral included in a rock ouarht not to be transferred to the lock 

 itself, as those who use Sorby's method are in a habit of doing. 



The somewhat common argument that a rock associated with crystalline 

 schists nuist have the same origin as the schists, would make a dike in slate 

 of the same origin as the slate. Association of rocks proves nothing, for in 

 volcanic districts, in limited areas even, rocks of every character can be 

 found together. Should we then hold that because some were sedimentary, 

 all the others were so ? Or, again, should we claim that because some were 

 eruptive, all the rest were eruptive ? No ! we ought to prove the origin of 

 each rock, and in every locality in which it occurs, so fiir as possible, and 

 when evidence is wanting leave the question as undetermined. 



* W'liitiicv's ^'()lcalloes, Earthquakes, and Mountain Buildiup:, pp^ 09-107. 



