xgoS.] THE PHYSICS OF THE EARTH. 263 



Again, in the " Voyage of the XaturaHst," Chapter XIV, he 

 adds : 



" The forces zvhich slowly and by little starts uplift continents, and those 

 zvhich at successive periods pour fortli I'olcanic matter from open orifices, 

 arc identical." 



It is unnecessary to dwell on the irresistible power which the great 

 naturalist correctly abscribed to volcanic and earthquake forces. It 

 is of more interest to notice that he declared them to be identical 

 with those which uplift continents. The same result is reached in 

 the present paper, about three quarters of a century later, and the 

 proof of the proposition now seems overwhelming. 



If Darwin had known the cause of seismic sea waves, and had 

 seen how trenches are dug out in the sea bottom by the expulsion 

 of lava, from beneath the sea under the land, can anyone doubt that 

 he would have discovered and proved the leakage of the oceans, 

 and developed the correct theory of mountain formation? 



§ 55. On the Oscillatory Movements of the Crust Shotvn in the 

 Coal Measures. — In view of the results established in this paper we 

 need not dwell on the coal measures, and other evidences of the 

 oscillation of the earth's crust. It suffices to say" that these oscilla- 

 tions actually took place, as geologists have long believed. The 

 coal fields in Pennsylvania were formed by vegetation growing 

 rapidly and with great luxuriance over areas near the sea level 

 which were again and again elevated and as often depressed by 

 earthquakes. When the land was under the sea the vegetation died 

 out, and mud and shale were deposited ; when the area was again 

 upraised another layer of vegetation was produced, and sometimes 

 it was deposited by floods, currents, and drifting where it had not 

 grown. This was during the Carboniferous Age, and while all the 

 land waS' near the level of the ocean. 



The details of such inquiries must be left to geologists and 

 paleontologists, who study the flora and fauna of past ages. Our 

 aim in these papers has been to give a firm basis for legitimate study 

 and speculation, without which the phenomena of nature remain 

 unintelligible. The progress of the sciences of the earth requires 

 two conditions : first, true physical causes ; and second, the intelli- 

 gent and consistent application of these causes to the explanation 



