58 C. II. HITCHCOCK — GEOLOGY OF OAHU 



The amount of volcanic material in proportion to the lime rock is 

 much greater as one ascends. My observation was not carried to a point 

 more than 100 feet above the sealevel, but it is evident that the sand 

 layers occur, interstratified in the mass, clear to the top (700 feet). The 

 upper limy layers appear, so far as observed, to be composed almost en- 

 tirely of calcareous sand, and no shells or corals were observed in them 

 in a recognizable state. At about 50 feet above the sea the heavy tuffs 

 overlie the uppermost heavy layers of calcareous rock. The latter is 

 nearly or quite horizontal, and consists of coral sand grains more or less 

 compactly consolidated, with occasional patches where marine fossil 

 shells were abundant. There are hardly traces of coral larger than fine 

 gravel and no coral masses. Where exposed the limestone has been 

 more or less subjected to solution by rain, and is usually covered by a 

 crust of the character previously mentioned, generally about an inch 

 thick. Below the upper and more sandy layers the rock is harder and 

 the sand grains smaller ; but the shells are more numerous and weather 

 out in a very perfect condition. The} r are mostly gastropods. Still 

 lower, casts and impressions of Ostrea and Avicula and coral heads less 

 than a foot in diameter are present. The bottom of these limestone 

 rocks is below the present sealevel. I saw nothing which could be re- 

 garded as an elevated reef of coral, but the rock presented the closest 

 analogy to what is usually designated " beach rock " in coral regions. 

 The conclusion to which I came was that the whole mass of Diamond 

 head had been slowly deposited in comparatively shallow water and 

 gradually elevated without being subjected to notable flexure. The 

 ejection of material at first must have been intermittent, with long quies- 

 cent periods, to enable the shore to have been repopulated with mol- 

 lusks and corals. The later layers may have been more frequently 

 ejected, as indicated by the absence of perfect fossils or of any fossils, 

 by the thinner calcareous and the heavier tufFaceous layers. 



The conditions appear to be incompatible with the reference of the 

 fossiliferous beds to a period as late as the Pleistocene. It is difficult 

 to make an exact comparison from the paleontological data, as the recent 

 fauna is still imperfectly known, and we have no standard of compari- 

 son in the whole Polynesian region by which the species could be com- 

 pared with those of Tertiary beds of known age; but the fossils have 

 every characteristic of those generally assigned to the Pliocene or upper 

 Miocene in their general aspect and state of fossilization. 



It is an interesting fact that the Achatinellida) have been recently 

 determined by Professor Pilsbry, of Philadelphia, from their anatomical 

 characters, to belong to a very primitive type of Pulmonata, analogous 

 to Limnsea, rather than the Bulimulus group, with which the} r have 



