22 THE OEIGIN AND ALTERATION OF ROCKS. 



care needs to be taken to prevent their being confounded with the super- 

 induced structures in the same rocks, for such mistakes are of very 

 frequent occurrence. 



Arguments from analogy should only be j^ermitted in deciding the ques- 

 tion of the origin of any rock when no otiier evidence exists ; and then such 

 arguments should be admitted to be of a doubtful character, and not held to 

 be proof positive. Anotlier prominent fallacy in petrography is the argument 

 that as the conditions are in one region, so must they be in another, — while 

 no effort is made to find out what the real conditions are in the latter. 



The origin of the older rocks, comprising the districts regarded as Azoic 

 or Arcliaian, and the principles on which they have to be subdivided into 

 groups, or ages, have an important bearing upon the classification of rocks. 

 The prevalent views regarding the constitution of these older formations 

 were looked upon, by the writer, as opposed to petrographical facts ; hence 

 it became necessary to make a careful examination of the published evi- 

 dence in behalf of these views. This has been done and duly published,* 

 wnth the result that no real evidence has been found sustaining the current 

 views; and thus the petrographer is free to follow the data of his science. 



The structure of the districts of crystalline rocks can in most cases be 

 explained in the following way. Let the reader imagine a region covered 

 by rock, either eruptive or sedimentary. Then suppose that here eruptive 

 (volcanic) action begins, and ashes, mud, lava, and the other accompaniments 

 of such action are mingled together. The earlier sedimentary rocks, and the 

 volcanic material are later cut through and through by dikes — fiuilted and 

 jointed, contorted, and if on a sea-shore, subjected to wave action before tiie 

 latter are f;iirly cold. 



We miii-ht thus have ming-led in inextricable confusion lava flows, indur- 

 ated and contorted sedimentary rocks, ashes, scoria, mud-flows, dikes, marine 

 deposits, in short, every known form of rock which can be produced by the 

 combined action of volcanic, pluvial, and marine agencies. After this, then 

 imagine the decline of the eruptive power, with the accompanying gaseous 

 and thermal water action. Ima(i;ine the chansj-es in the rock structure that 

 these agents would produce, as previously mentioned imder rock alteration, 

 including vein phenomena ; and then consider wliat would be the effect upon 

 sucli a district of the action of every conceivable geological agency during 

 the countless years since the early geological ages. Then suppose that some 



* Bull. Mus. Coiiip. Zuiil., ISSO, vii. No. 1; ISSi, No. U. 



