412 Metamorjphism. 



connect with the nervous system. Those currents of earth 

 magnetism, obtained in some mysterious way from the sun, and 

 passing from pole to pole of the earth, both above and within 

 the surface : — recognized in the air by one kind of phenomena, 

 and now well recognized in the substance of the earth by their 

 effects on the telegraph wire : — those electric forces induced by 

 and governing chemical change, such as the passage of currents 

 of water through different minerals cannot fail to produce ; all 

 these are the representatives of nervous energy in the earth, 

 while all, so far as we know, are strictly superficial phenomena. 

 Lastly, there is vital heat, represented in the earth also by the 

 higher and equable temperature at moderate depths. 



These inquiries have nothing to do with doubtful questions 

 as to the earth's nucleus and its temperature now, or at any pre- 

 vious period; they are fair questions for discussion, for they admit 

 of experimental investigation, and they tend to place geological 

 science on its right footing, apart from mere idle theory and 

 baseless assumption. Thus we know that water can traverse 

 all rocks, and we have the best possible reason for believing 

 that it does so. We know that it is capable of dissolving almost 

 every element, and that it acts energetically on most rocks when 

 containing certain acids which are constantly provided. We 

 know that earth- magnetism is a reality, and that magnetic and 

 other forms of electricity are convertible ; we can hardly doubt 

 also that heat and electricity and chemical action are mutually 

 convertible, and in the strictest sense are co-relative. We 

 know that chemical action induced by the passage of water 

 under certain circumstances must involve changes of tempera- 

 ture, and that the increment of temperature thus produced may 

 be very great. There is really no need for the assumption of a 

 central fire, even to account for the steady increase of tempera- 

 ture observed at increased depths. We have no right to intro- 

 duce the Beus ex macJmid when the ordinary machinery is 

 sufficient, and so long as we can explain metamorphism by 

 methods of nature that we understand and can copy ; so long, 

 in a word, as water will serve our purpose, it is certainly not 

 philosophical, nor is it altogether reasonable, to allow our- 

 selves to be prejudiced by vague resemblances, which, when 

 traced to their foundation, are found to be little more than 

 analogies. 



Metamorpliism, then, is a great fact in nature. Every 

 fragment of limestcme, sandstone, and slate, or shale, that enters 

 into the composition of the rocks of the earth, is more or less 

 metamorphosed; every fossil that is found bedded in rocks is 

 likewise changed, and shows metamorphism. It is not only 

 the granites and other crystalline roeks that have been so much 

 altered that their original texture and history are lost sight of; 



