CHAPTER VII. 



STRUCTURAL OR TECTONIC FEATURES OF ROCK MASSES. 

 ORIGINAL STRUCTURES AND LITHOGENESIS OF THE PYRO- 

 GENIC ROCKS. 



When rock masses are considered on a large scale, certain struc- 

 tural features are seen which furnish a clue to the geological his- 

 tory of the region. These can generally be observed only in the 

 field. Two classes of such structural or tectonic features are recog- 

 nized : (i) original structures and (2) secondary or later struc- 

 tures. To the first belong all those accompanying the phenomena 

 of rock formation, features of essential significance in the study of 

 the origin of the formation which they characterize. To the second 

 belong all structures due to subsequent disturbances such as faults, 

 folds, cleavage, and the like, as well as metamorphism, or the alter- 

 ation of rocks. While all these need to be carefully considered in 

 working out the stratigraphy of any region, they have not the im- 

 portance possessed by the original structures, which alone furnish 

 clues to the method of formation of the rock possessing them. In 

 this and the next eleven chapters the Original Structures of rocks 

 will be discussed ; the secondary ones will be taken up in Chapters 

 XIX and XX. Since each class of endogenetic rocks has its own 

 mode of origin, the original structures of each must be of corre- 

 sponding individuality. 



THE PYROGENIC ROCKS. 



Pyrogenic or igneous rock masses naturally fall into two great 

 divisions, (a) the intrusive and (b) the effusive. The former 

 solidify within the crust of the earth, appearing as a part of the 

 surface only through subsequent erosion of the overlying masses, 

 while the latter reach the surface in a molten condition, and on 

 solidifying form the uppermost crust of the earth at that point. 

 In otherwise undisturbed regions intrusive igneous masses are al- 

 ways younger than the enclosing rocks, while efifusive igneous 



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