CHAPTER X 

 MATERIALS OF THE EARTH AND THEIR ARRANGEMENT 



The general constitution of the lithosphere has been referred 

 to already (p. 7), but we are now to study in more detail the 

 nature, the arrangement, and the history of the rocks. The igneous 

 rocks will be considered first. 



IGNEOUS ROCKS 



Appearance at the surface. The preceding chapter has acquainted 

 us with the fact that some igneous rocks were extruded either from 

 volcanoes or from fissures, and that extrusive rocks include both 

 lava flows and pyroclastic materials. Under proper conditions, 

 extruded rocks may be buried later beneath sediments, or may be 

 worn away by erosion. It follows that only a part of the igneous 

 rocks extruded in the past, and especially those of relatively recent 

 times, remain at the surface. 



By removing the overlying rocks, erosion exposes the intruded 

 rocks of dikes, sills, laccoliths, batholiths (p. 228), etc., and a 

 considerable part of all accessible igneous rock is now at the surface 

 because the rocks which overlay it have been worn away. The 

 great areas of granite in Canada, and the long axes of many of our 

 western mountains, are examples. Extruded igneous rock which 

 has been buried, also is subject to subsequent exposure by the 

 wasting away of its cover. 



Structural features of igneous rocks. The names applied to the 

 principal forms of igneous intrusions imply certain large structural 

 features; but igneous rocks have certain other structural features 

 which distinguish them from other rocks. Thus the rock of lac- 

 coliths, bysmaliths, and batholiths is generally massive. This 

 term means not simply that the rock occurs in large bodies, but 

 that the rock has no distinct cleavage. It is not in beds, and it is 

 not schistose. Sills and some extrusions of lava take on the form 

 of sheets. Where one extrusive sheet of lava overlies another, the 

 succession of sheets has some resemblance to stratified rock; but 



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