April, 1834. 



BASALTIC PLATFORM. 



217 



any part, these examples are most striking of the inefficiency 

 of rivers in transporting even moderately-sized fragments. 



The basaltic cliffs are obscurely divided by lines of more 

 cellular or amygdaloidal varieties, and the strata appear to 

 the eye perfectly horizontal. They overlie the great tertiary 

 deposits, and are covered (except where denuded in some 

 of the lower terraces) by the usual beds of gravel. The 

 basalt is clearly nothing more than lava, which has flowed 

 beneath the sea; but the eruptions must have been on the 

 grandest scale. At the point where we first met this form- 

 ation, the mass was about 120 feet in thickness; following 

 the river-course, it imperceptibly rose and became thicker, 

 so that at forty miles above the first station it was 320 

 feet. What the thickness may be close to the Cordillera, I 

 have no means of knowing, but the platform there attains 

 an elevation between two and three thousand feet above 

 the level of the sea : we must therefore look to the moun- 

 tains of that great chain for its source ; and worthy of such a 

 source are streams, that have flowed over the bed of an ocean 

 to a distance of one hundred miles. 



A fine section of the basaltic platform is presented by the 

 cliff's on both sides of the valley. At the first glance it is 

 evident the strata must at one time have been united. What 

 power then has removed along a whole line of country, a 

 solid mass of very hard rock, which had an average thickness 

 of about three hundred feet, and a breadth varying from rather 

 less than two to four miles ? The river, though it has so 

 little power in transporting even inconsiderable fragments, 

 yet in the lapse of ages might produce an efl'ect by its gra- 

 dual erosion, of which it is difficult to judge the limit. But 

 in this case, independently of the insignificance of such 

 agency, good reasons can be assigned for believing that this 

 valley was formerly occupied by an arm of the sea. It is 

 needless in this work to detail arguments, which chiefly rest 

 on the form and nature of the banks, on the manner in which 

 the valley near the foot of the Andes expands into a great 

 bay, and on the occurrence of a few sea-shells lying in the 



