742 ROCKS ALTERED BY SUBTERRANEAN GASES. [Ch. XXXV. 



doughy mass. He conceives, therefore, that steam in the bowels of 

 the earth having a temperature equal or even greater than the melt- 

 ing point of lava, and having an elasticity, of which even Papin's 

 digester can give Trot a faint idea, may convert rocks into liquid 

 matter.* 



The above observations are calculated to meet some of the objec- 

 tions which have been urged against the metamorphic theory 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 contact 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. 757, p. 738.) This is undoubtedly an ex- 

 treme case ; but it is natural to suppose that analogous causes may, 

 under favorable circumstances, affect masses of greater volume. The 

 metamorphic theory does not require us to affirm that some contigu- 

 ous mass of granite has been the altering power; but merely that an 

 action, existing in the interior of the earth at an unknown depth, 

 whether thermal, hydrothermal, or other, analogous to that exerted 

 near intruding masses of granite, has, in the course of vast and in- 

 definite periods, and when rising perhaps from a large heated surface, 

 reduced strata thousands of yards thick to a state of semifusion, so 

 that on cooling they have become crystalline, like gneiss. 



The prominent part which water has played in distributing the 

 heat of the interior through mountain masses of incumbent strata, 

 and in conveying various mineral elements in a fluid or gaseous state 

 into the same masses, so as to give rise in the course of long geologi- 

 cal periods to vast chemical changes, enables us to dispense with the 

 intense heat formerly thought necessary for the production of the 

 metamorphic rocks. But, on the other hand, the length of time 

 which must have been consumed during the escape of so much heat 

 from molten matter underlying the solid crust, at the depth of many 

 miles, raises our conception of the great original intensity of temper- 

 ature required to bring those subterranean sheets of lava into a liquid 

 state. That they are sometimes of vast horizontal extent, even hun- 

 dreds of miles in length, seems proved by facts observed during erup- 

 tions in the volcanic region of the Andes. 



The scorching heat radiated by lava in a volcanic crater, when it 

 is white and glowing like the sun, prepares us to believe that the 

 temperature of the same fluid thousands of fathoms below, must far 

 exceed any heat which can ever be witnessed at the surface. The 

 uniform composition, the absence of stratification, and the great vol- 



* Jam. Ed. New Phil. Journ., No. 51, p. 43. 



