CHAPTER X. 



THE RELATIONS OF METAMORPHISM TO STRATIGRAPHY. 



INTRODUCTORY. 



It is sufficiently evident from the previous chapter that rocks which 

 have very similar lithological characters may have had very different 

 origins. This point can probably best be appreciated with the aid of a few 

 illustrations. 



(a) It is well known that often it is difficult or impossible to discrim- 

 inate limestones of org-anic and chemical origin. 



(b) Sometimes true water-deposited conglomerates and volcanic tuffs 

 closely resemble each other, and often they grade into each other. This 

 last occurs wherever volcanoes are adjacent to the sea and the fragmental 

 material falls partly upon the land and partly in the water. 



(c) A disintegrated and partly decomposed granite or other massive 

 rock in situ is often very similar to an arkose which is formed from it. For 

 instance, it has been pointed out that the arkoses which are forming in the 

 Gulf of California are practically identical in mineral and chemical char- 

 acter with the rocks from which they are formed. In western United 

 States at various places, even where there is very little alteration, in the 

 field it is difficult to delimit sharply the massive rocks, such as granite, 

 from the arkoses built up from and resting upon them. 



(d) It is frequently difficult to determine whether igneous rocks are 

 tuffs or are lavas broken during fiowag*e. Gradations between these two 

 classes of rocks occur where tuffaceous material falls upon unconsolidated 

 lavas. 



If rocks comparatively little modified and of different origin may so 

 closely resemble one another that they are difficult to discriminate, it is 

 plain that after profound metamorphism they may be still nearer alike. In 



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