EXPLOraNG THE SANTA CRUZ. 231 



April 26t7i. — We this day met with a marked 

 change in the geological structure of the plains. 

 From the first starting I had carefully examined 

 the gravel in the river, and for the two last days 

 had noticed the presence of a few small pebbles 

 of a very cellular basalt. These gradually in- 

 ci'eased in number and in size, but none were as 

 large as a man's head. Tliis morning, however, 

 pebbles of the same rock, but more compact, sud- 

 denly became abundant, and in the course of half 

 an hour we saw, at the distance of five or six miles, 

 the angular edge of a great basaltic platform. 

 When we arrived at its base we found the stream 

 bubbling among the fallen blocks. For the next 

 twenty-eight miles the river-course was encumber- 

 ed with these basaltic masses. Above that limit im- 

 mense fragments of primitive rocks, derived from 

 the surrounding bouldcr-forraation, were equally 

 numerous. None of the fragments of any consid- 

 erable size had been washed more than three or 

 four miles down the river below their parent-source: 

 considering the singular rapidity of the great body 

 of water in the Santa Cruz, and that no still reach- 

 es occur in any part, this example is a most stri- 

 king one, of the inefficiency of rivers in transport- 

 ing even moderately-sized fragments. 



The basalt is only lava, which has flowed be- 

 neath the sea; but the eruptions must have been 

 on the grandest scale. At the point where we first 

 met this formation it was 120 feet in thickness; 

 following up the river course, the surface imper- 

 ceptibly rose and the mass became thicker, so that 

 at forty miles above the first station it was 320 feet 

 thick. What the thickness may be close to the 

 Cordillera I have no means of knowing, but the 

 platform there attains a height of about three thou- 

 sand feet above the level of the sea : we must 



