148 LYELL'S ELEMENTS OF GEOLOGY. 



Origin of Metamorphic Structure. 



and their heating and modifying influence may be spread through- 

 out the whole of the solid mass. 



The above observations are calculated to meet some of the 

 objections which have been urged against the metamorphic the- 

 ory on the ground of the small power of rocks to conduct heat; 

 for it is well known that rocks, when dry and in the air, differ 

 remarkably from metals in this respect. It has been asked how 

 the changes which extend merely for a few feet from the con- 

 tact of a dike could have penetrated through mountain masses 

 of crystalline strata several miles in thickness. Now it has been 

 stated that the plutonic influence of the syenite of Norway, has 

 sometimes altered fossiliferous strata for a distance of a quarter 

 of a mile, both in the direction of their dip and of their strike. 

 (See Fig. 126. p. 144.) This is undoubtedly an extreme case; 

 but is it not far more philosophical to suppose that this influence 

 may, under favourable circumstances, affect denser masses, than 

 to invent an entirely new cause to account for effects merely dif- 

 fering in quantity, and not in kind ? The metamorphic theory 

 does not require us to affirm that some contiguous mass of gra- 

 nite has been the altering power ; but merely that an action, ex- 

 isting in the interior of the earth at an unknown depth, whether 

 thermal, electrical, or other, analogous to that exerted near in- 

 truding masses of granite, has, in the course of vast and indefi- 

 nite periods, and when rising perhaps from a large heated sur- 

 face, reduced strata thousands of yards thick to a state of semi- 

 fusion, so that on cooling they have become crystalline, like 

 gneiss. Granite may have been another result of the same ac- 

 tion in a higher state of intensity, by which a thorough fusion 

 has been produced ; and in this manner the passage from granite 

 into gneiss may be explained. 



Some geologists are of opinion, that the alternate layers of 

 mica and quartz, or mica and felspar, or lime and felspar, are 

 so much more distinct in certain metamorphic rocks, than the 

 ingredients composing alternate layers in many sedimentary de- 

 posits, that the similar particles must be supposed to have exert- 

 ed a molecular attraction for each other, and to have thus con- 

 gregated together in layers, more distinct in mineral composition 

 than before they were crystallized. 



In considering, then, the various data already enumerated, the 

 forms of stratification in metamorphic rocks, their passage on the 

 one hand into the fossiliferous. and on the other into the plutonic 

 formations, and the conversions which can be ascertained to have 

 occurred in the vicinity of granite, we may conclude that gneiss 

 and mica-schist may be nothing more than altered micaceous 

 and argillaceous sandstones, that granular quartz may have been 



