GEOLOGY AND TOPOGRAPHY OF HAAVAII. 115 



was, a small fresh water lake. 200 feet above the sea, thnt was frequented ])y wild 

 fowl at the proper season. 



Dr. Sereno E. Bishop made Diamond Head the basis of a study calculated 

 to show the brief time required for the completion of tuff-cones of similar form. 

 He concluded that such a cone "could have Ixmmi crcati'd only hy ;m cxti'cmely 

 rapid projection aloft of its material, comi)leted in a few liours at the most, 

 and ceasing suddenly and finally." Taking into account the extreme regularity 

 of its rim and the uniform dip and character of its crater he i)r()ceeded, with a 

 mathematical calculation, to estimate that the 18,000,000,000 cubic feet of ma- 

 terial that forms its mass could have been raised to approximately 12,000 feet, 

 and dropped into its present position in two liours' time, and he was inclined to 

 increase the velocity of the ejecta and reduce the time to perhaps one hour 

 Other geologists, however, are very likely to question the soundness of the con- 

 clusions drawn by Dr. Bishop since there is unmistakable evidence that it was 

 in eruption a number of times with intervening periods of repose. 



Punchbowl Hill. 



Punchbowl Hill, with a form which suggests its name — lies just back of the 

 city and is 498 feet high. It is similar to Diamond Head in form and structure 

 and has in its outer wall on the town side, numerous seams filled with calcite. 

 Much can be learned of the geology of the vicinity by the study of the cone 

 itself and from the phenomena about it. Other tuff-cones are Tantalus, Salt 

 Lake, and Koko Head ; there are still others on the opposite side of the island at 

 Kaneohe, as well as at the south end of the Waianae ]\Iountains at Laeloa. 

 Some of the cones in the latter region, however, are small basaltic craters, as 

 are also the one on Rocky Hill in ]\Ianoa A^alley, and the two small craters, 

 IMuumai and Kaimuki. on the ridge l)ack of Diamond Head, to the east of Hono- 

 lulu. 



Elevated Goral Reefs. 



Almost the entire shore-line of Oahu shows more or less evideiiee of elevated 

 coral reefs. In the vicinity of Honolulu these reefs form the foundation on 

 which much of the city it built. The elevated reefs are most extensive, how- 

 ever, in the vicinity of Pearl Lochs, where they are intinuitely associated with 

 the sedimentary deposits, volcanic flows, decaying rock and volcanic ash. It is 

 thought by Professor Hitchcock and others that this series of deposits began 

 in the Pliocene period and that it and the older layers beneath may be a base on 

 v.hich the ejections that formed the volcanic island began to accumulate as 

 indicated on Plate 75. The region about Pearl Harbor is one of much geologic 

 interest, but is far too complicated in eharacter to l)e readily interpreted by the 

 casual visitor. Features of general interest, however, are that in many places 

 as many as nine or ten stratified deposits may he seen in a vertical cut of forty 

 or fifty feet, and that in the region, beds from one to tlii-ee or four feel thick, of 

 large oyster shells (Ostrea retusa) are exposed, far inland. Aeeoi-ding to tlie in- 

 vestigations of Professor Hitchcock, "the Pliocene area of Oahn eoineides very 



