the Igneous Platform. 197 



from 25 to 48 inclusive, is seen in number 38, which, in addi- 

 tion to consisting of the tiny, angular chips characteristic of a 

 triturated rock, contains also grains which are distinctly rounded 

 as by water ; and such grains are of a different color and 

 appearance, being light gray to brown, and distinctly oxidized. 

 It is assumed, therefore, that in this stretch from 1,045-1,075 

 feet a bed of sand was encountered, showing deposits which 

 had been worn upon a beach. 



Geological Conclusions. 



The most striking general feature of the series of samples of 

 volcanic material below the limestones, which end at 250 feet 

 below sea-level, is the thoroughly and deeply oxidized condi- 

 tion of those above 600 feet in depth, compared with the little 

 or entirely unoxidized nature of those below this point. They 

 are all of them far below the present sea-level, so that they 

 appear to be subject to similar conditions so far as their present 

 situation is concerned. Nor can we find an explanation in the 

 idea that this difference may be due to structure of some kind ; 

 that the oxidized material represents originally fragmental vol- 

 canics, while the unoxidized is from solid lava flows, because 

 in number 18 we find a thin layer of unoxidized fragmental 

 material lying in the midst of the oxidized zone, and below 

 600 feet we first pass through a considerable layer of unoxi- 

 dized fragmental sandy deposit before striking the solid lava. 

 This makes it evident that the material of the oxidized zone 

 has, at some former period in its history, been subjected to 

 conditions quite different from those now obtaining, and which 

 have affected the lower zone. The brown color and earthy 

 appearance at once suggest the WT>rk of the atmosphere, and 

 the contrast between the two that the lower zone, having been 

 always below sea-level, its rocks have been protected from 

 weathering, and have been subjected only to such action as sea- 

 water is able to exert. This action, so far as the writer has 

 been able to discover, has not been definitely studied. There 

 seems to be general agreement, however, that rocks which are 

 protected by sea-water from the atmosphere will not suffer 

 oxidation and may even remain unchanged. More than half 

 a century ago J. D. Dana* observed the protective effect of 

 sea-water in preventing the oxidation of basalt on the south 

 coast of Australia. He says "it is a general and important 

 fact that a rock which alters rapidly when exposed to the united 

 action of air and water, is wholly unchanged when immersed 

 in water, or exposed to a constant wetting by the surf." In 

 commenting on this, Merrillf says : " While no exception can 



*Keports of the Wilkes Exploring Expedition, Geology, p. 514. 

 f G, P. Merrill, Eocks, Kock-Weathering, and Soils, p. 239. 



