54 C.H.HITCHCOCK — GEOLOGY OF OAHU 



by Doctor Walter Maxwell for the Hawaiian islands, to whose work the 

 reader is referred for full information.* 



Fossil Land Shells 



At the western base of Diamond head I have gathered many speci- 

 mens of the genera Achatinella and Helix in a breccia consisting of tuff 

 cemented by calcium carbonate, the latter mineral having been derived 

 from the underlying reef when the tuff was ejected. The order of gen- 

 esis is : 



1. The deposition of the coral reef upon an ancient lava. 



2. The ejection of tuff through an opening in the reef produced by seismic 

 action, probably starting in shallow water and bringing up samples of the lava 

 and limestone. 



3. The crater of Diamond head supported vegetation on its flanks above the 

 sealevel, and while thus situated fragments of the volcanic rock rolled down the 

 steep sides, water washed the lime into the talus, cementing the pieces, and the 

 land shells contributed their shells to the pile. 



4. A time of submergence succeeded, certainly to the amount of 40 feet — some 

 say 200. 



5. The land then rose to its present level. 



The breccia is fully 25 feet thick, and hence a considerable time was 

 required for its accumulation. At a quarry it is easy to divide the 

 strata into three parts, in the upper two of which the shells are found. 

 The locality was first described by Professor A. B. Lyons. t In a recent 

 letter he says : « 



" It seems certain that the shells lived on the mountain and were not brought 

 down in streams from a distance. The vegetation must have been more abun- 

 dant than now, because the shells are such as are now found in moist localities. 

 As to the species, at least 'five genera are represented, all such as have their rep- 

 resentatives in the valleys of Oahu, though some of the species may be extinct. 

 The genera included Lepachatina, three species; Hellcona, one; Pitys, one; Suc- 

 cinea, one (like one now living on the head); Pupa, I think one, and the old Helix 

 lamblata. We should not look for any great antiquity in secondary formations 

 from materials of one of the tufa cones of Oahu. The absence of marine shells 

 from the mass is significant. Submergence must have taken place later." 



Specimens from the middle and upper part of the talus-breccia were 

 submitted to Doctor J. T. Gulick, with the query whether any altera- 

 tions had been made in the later growths, having in mind evolutionary 

 changes. He said that the upper ones vary from those below by being 



* Lavas and soils of the Hawaiian islands, Honolulu, 1898. See also A. B. Lyons : Chemical com- 

 position of Hawaiian soils and of the rocks from which they have been derived. Am. Jour. Sci., 

 December, 1896. 



t Thrum's Annual for 1891, p. 103/ 



