PART II. CHAPTER XXV. 



299 



Age of Metamorphic Rocks, and Order of Superposition. 



certain depth beneath the surface, they must always, before they 

 are upraised and exposed at the surface, be of considerable anti- 

 quity, relatively to a large portion of the fossiliferous and vol- 

 canic rocks. They may be forming at all periods ; but before 

 any of them can become visible, they must be raised above the 

 level of the sea, and some of the rocks which previously con- 

 cealed them must have been removed by denudation. If the 

 student will refer to the frontispiece, he will see that the strata 

 A, which were the last deposited, are every where hidden from 

 human observation by the sea, while the contemporaneous meta- 

 morphic rocks C are concealed at a still greater depth, as are 

 also the plutonic rocks D of the same age. He will also observe 

 that the strata C, which have recently become metamorphic, are 

 not parts of A, nor even of the groups immediately antecedent 

 in date a, b, c, but they are portions of much older formations, 

 d, e, f, g, h, i. Now, suppose that part of the earth's crust, 

 which is represented in the frontispiece to be subjected, in vari- 

 ous places, to a long series of upheaving and depressing move- 

 ments ; the beds A will, here and there, be partially upraised 

 and converted into dry land, but the hypogene rocks C, D, al- 

 though brought up nearer to the surface, will still, very probably, 

 remain hidden from sight. Let a second period elapse, and the 

 rocks A may be raised in some countries to a height of several 

 thousand feet ; and still the rocks C and D may be almost every 

 where hidden. During a third period, when the stratified form- 

 ations A have been laid dry over large continental areas, and 

 have reached the summits of some Alpine chains, the hypogene 

 formations C D may also be forced up and exposed to view above 

 the level of the ocean by similar causes ; but they will rank no 

 longer as modern rocks, the geologist being already acquainted 

 with newer groups, both fossiUferous and volcanic. The student 

 will also perceive how impossible it may then be to prove that 

 the strata C became metamorphic at the period of the deposition 

 of A, and how difficult not to exaggerate the antiquity of C as a 

 series of metamorphic rocks, when the remote period of their 

 deposition has been ascertained, and the comparatively modern 

 era of their crystallization remains uncertain. 



Order of succession in Metamorphic rocks. — There is no 

 universal and invariable order of superposition in metamorphic 

 rocks, although a particular arrangement may prevail through- 

 out countries of great extent, for the same reason that it is trace- 

 able in those sedimentary formations from which crystalline 

 strata are derived. Thus, for example, we have seen that in the 

 Apennines, near Carrara, the descending series, where it is 



