chap. xt. of South America. 597 



of high activity in the neighbouring volcanos. During 

 the cretaceo-oolitic period this certainly appears to have 

 been the case at the Puente del Inca, judging from the 

 number of intercalated lava-streams in the lower 3,000 

 feet of strata ; but generally, the volcanic orifices seem 

 at this time to have existed as submarine solfataras, and 

 were certainly quiescent compared with their state 

 during the accumulation of the porphyritic conglomerate 

 formation. During the deposition of the tertiary strata 

 we know that at S. Cruz, deluges of basaltic lava were 

 poured forth ; but as these lie in the upper part of the 

 series, it is possible that the subsidence may at that 

 time have ceased : at Chiloe, I was unable to ascertain 

 to what part of the series the pile of lavas belonged. 

 The Uspallata tuffs and great streams of submarine 

 lavas, were probably intermediate in age between the 

 cretaceo-oolitic and older Tertiary formations, and we 

 know from the buried trees that there was a great sub- 

 sidence during their accumulation ; but even in this 

 case, the subsidence may not have been strictly con- 

 temporaneous with the great volcanic eruptions, for we 

 must believe in at least one intercalated period of eleva- 

 tion, during which the ground was upraised on which 

 the now buried trees grew. I have been led to make 

 these remarks, and to throw some doubt on the strict 

 contemporaneousness of high volcanic activity and 

 movements of subsidence, from the conviction impressed 

 on my mind by the study of coral formations, 1 that 

 these two actions do not generally go on synchronously ; 

 — on the contrary, that in volcanic districts, subsidence 

 ceases as soon as the orifices burst forth into renewed 

 action, and only recommences when they again have 

 become dormant. 



1 The Structure, &c. of Coral Reefs, 1st ed. p. 140 ; 2nd ed. p. 180. 



