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This author holds the non-uniformitarian view with regard to igneous 

 phenomena. He considers that the general conditions of eruption upon the 

 planet have appreciably changed during the period of time covered by our 

 fossiliferous deposits ; and that the differences observed between the igneous 

 products of different periods are due, in a great measure, to this change. In 

 considering the general question of the connection between age and the other 

 characters of igneous rocks it will be convenient to bear in mind the views 

 of this author. 



In dealing with the subject it is important to distinguish between 

 original and secondary characters. Rocks of different ages, which are 

 identical so far as original characters are concerned, may differ considerably 

 in consequence of alteration. A difference of this kind, though it may be of 

 use in classification, does not of course indicate any variation in the character 

 of igneous phenomena. The important question we have first to ask ourselves 

 then is this : Are there any marked differences between the igneous rocks of 

 different periods so far as original characters are concerned ? The answer to 

 this question will depend very largely on our method of treating the subject 

 and on the extent of our field of view. 



Igneous rocks may consolidate at the surface in the form of lavas and 

 f ragmental rocks ; near the surface in veins and dykes ; and at considerable 

 depths beneath the surface in large masses. Rocks which form at the surface 

 have a certain set of characters ; those which form beneath the surface have 

 another set of characters. The rocks of the former group are often 

 scoriaceous, vesicular, amygdaloidal and porphyritic. They frequently 

 contain more or less amorphous matter and are associated with fragmental 

 rocks of similar composition. The rocks of the latter group are usually solid 

 throughout, coarse in grain, granular in texture and holo-crystalline. They 

 are never associated with contemporaneous fragmental rocks. Now in 

 comparing the igneous rocks of different periods with each other for the 

 purpose of determining whether any change has occurred in the character 

 of igneous phenomena during geological time, it is obviously necessary that 

 the comparison should be instituted between rocks of the same group. The 

 importance of this will appear at once when we reflect that the plutonic 

 rocks associated with the older must necessarily be more numerous than those 

 associated with the younger formations, even if igneous action has been 

 uniform throughout geological time. This must be so for two reasons : 

 (1) the plutonic (deep-seated) rocks can never be directly associated with 

 contemporaneous stratified deposits ; and (2) they can only be exposed by 

 the denudation of the overlying rock, and therefore not until a long time 

 has elapsed since their formation. 



Let us now compare the volcanic rocks of different periods with each 

 other, and endeavour to ascertain what differences occur between them. 

 Our views on the subject of the connection between the composition and 

 texture of igneous rocks and geological age will vary with the extent of 

 our field of view. If we limit our attention to a small area of the earth's 

 surface and to a small portion of geological time, then it becomes possible 

 to establish a connection between the geological age and the petrographical 



