Crosby.] 450 [November 7, 



century, or even from decade to decade. And the evidence is 

 very clear that this movement has been going on for a long time 

 over the whole southern half of the continent, but especially on 

 the west side. From Cape Horn northward, 1,200 miles on the 

 east coast and nearly 2,500 miles on the west coast, old sea- 

 beaches and terraces full of species of shells now living in the 

 adjacent seas, are observed at heights ranging from 100 to 1,300 

 feet above the sea. These are the facts furnished by Darwin, 

 but more recently Alexander Agassiz has traced the ancient 

 shore line in Peru at an elevation of 3,000 feet by means of coral 

 still sticking to the rocks. There is also evidence that points dis- 

 tant from the coast rise faster than the coast itself. 



Here are indisputable facts which demand a plastic zone under 

 a large section of South America at the present time. And, by 

 presenting the similar facts observed in other continents, it 

 would be easy to show that a very large part of the land of the 

 globe is now moving up or down, and hence must be now under- 

 laid by a plastic stratum. While, on the other hand, there is not 

 a vestige of trustworthy, positive evidence to show that this plas- 

 tic stratum is not as wide-spread and continuous to-day as it ever 

 was ; and the probabilities certainly are that it is nearly so. That 

 is, contrary to the ideas inculcated by some leading geologists, I 

 venture to assert that we have no evidence that the conti- 

 nents are more stable now than they were during the Tertiary or 

 Secondary periods. 



Among recent geological writers of note, there are none who 

 are more pronounced believers in a thoroughly solid earth than 

 Archdeacon Pratt and Prof. Joseph Le Conte. They both ap- 

 pear to deny the existence of a plastic zone, considering the earth 

 as entirely solid, with the exception of a few limited and isolated 

 lakes of liquid rock situated in the crust and forming the sources 

 of volcanic materials. Rejecting the idea of a plastic zone, Le 

 Conte is, as he confesses (Araer. Jour. Sci. (3), vol. iv, p. 472), 

 unable to explain the phenomena of elevation and subsidence. 

 Le Conte is an able and prominent advocate, and in part the au- 

 thor, of the theory of the formation of mountains which has been 

 developed during the last score of years — chiefly by Hall, Hunt, 

 Dana, and Le Conte — and which is at present very generally ac- 

 cepted by geologists. The most prominent and essential feature 



