463 Prof. Sedgwick on the Structure of large Mineral Masses. 



The original modifications in the structure of an igneous rock mai/ have 

 been produced in a comparatively short period of time ; and the same remark 

 applies to some jnetamorphic rocks. The saccharoid texture (for example) of 

 limestone, when in contact with trap, ma7/ have been produced during- a very 

 short period ; for we know that this effect has been beautifully imitated in a 

 chemical laboratory. In general, when metamorphic rocks appear to have 

 been in a state of igneous fusion, it is obvious that all questions, respecting- 

 the length of time during which their crystalline structure was elaborated, 

 must come very nearly under the rule that affects igneous rocks. 



There is, however, a large class of metamorphic rocks, the structure of 

 which can only have been produced by causes acting during long- periods of 

 time. I am not now speaking- of gneiss, mica slate, and other old formations of 

 crystalline strata. To assume that all such rocks are 77ieta??iorphic is nothing 

 better than to beg some of the greatest fundamental questions in geology. 

 But in cases where a new mineral structure appears certainly to have been 

 superinduced by direct igneous action, we sometimes meet with phasnomena 

 utterly at variance with the hypothesis of a chemical action continued only 

 through a short period. Rocks, it is well known, are bad conductors of heat; 

 yet among stratified rocks the manifestations of igneous action are sometimes 

 propagated to great distances. Such phaenomena may be readily explained. 

 Masses of granite and porphyry were not necessarily protruded instanta- 

 neously. They may have been many years in assuming their present relative 

 position among the stratified formations. Again, the effect produced by such 

 protruded masses might be modified, almost indefinitely, by the conducting 

 powers of the materials among which they rose. One mass may have been 

 pushed out into the sea or the open air; another, after its first elevation, may 

 still have been covered up by a vast thickness of badly conducting strata. 

 Nor is this all. It is by no means necessary to suppose that all changes pro- 

 duced by igneous agents on stratified rocks took place only during periods of 

 eruption. We may suppose, for example, that the lower slate formations of 

 Cornwall and Cumberland formed a dome, overhanging the great subterranean 

 fires, for many years, or even for many centuries, before a contraction of the 

 upper surface, or a mechanical action from below, pushed up the great bosses 

 of granite among the altered and half-molten beds. Hence, although it be 

 certain that the structure of altered rocks has in many cases been produced 

 by a sudden action, we are by no means limited in our hypothesis, but may 

 fairly suppose such periods of duration as in each case, are necessary to the 

 elaboration of our phaenomena. 



My chief object is, however, to describe some of the changes produced on 



