282 



THE WONDERS OF GEOLOGY. Lect. III. 



displayed, so largely is this marl composed of the skeletons 

 or shields of animalcules ; in fact, very few inorganic par- 

 ticles enter into its composition.* 



57. Altered tertiary strata of the Andes. — But 

 striking as are the proofs already adduced of elevations and 

 subsidences, and other effects of volcanic agency during the 

 tertiary period, they sink into comparative insignificance, 

 when contrasted with the enormous changes which have 

 taken place in the great mountain chains of South America, 

 during the same geological epoch. From the observations 

 of Mr. Charles Darwin, we learn that an extensive tertiary 

 system, analogous to that of Europe, skirts both flanks of the 

 primary rocks which form the southern chain of the Andes ; 

 the latter having suffered a certain degree of elevation be- 

 fore the deposition of the former. These strata, which are of 

 great thickness and extent, are separable into two groups ; 

 the lowermost beds, like those of Auvergne, repeatedly alter- 

 nate with lavas, and thus denote the commencement of the 

 eruptions of the ancient craters. Over these are accu- 

 mulations of porphyritic pebbles, covered, at elevations of 

 many hundred feet, by beds of shells of recent species ; and 

 the sides of the mountains appear like a succession of sea- 

 beaches, which have been slowly and tranquilly lifted up. 

 The altered character of the tertiary deposits within the 

 influence of the igneous products, — the conversion of beds 

 of loose pebbles into solid, compact rocks, — and the occur- 

 rence of metalliferous veins in strata of comparatively 

 modern origin, — are facts so powerfully exemplifying the 

 geological principles enunciated in the former lectures, that 

 though this discourse has extended to a great length, I 

 cannot omit Mr. Darwin's graphic description of these 

 phenomena, as originally communicated in a letter to Pro- 

 fessor Henslow, of Cambridge, dated Valparaiso, March 1835. 



* Sec the Article " Fossil Animalcules" in Medals of Creation, 

 vol. i. pp. 224—2-28. 



