GEOLOGY AND TOPOGRAPHY OF HAAVAII. 121 



short distance over the elevated reef at :\loiliiH. Rocky Hill must have been in 

 eruption after the reef was formed. 



Next came the period of the eruption of the tutf craters: the Salt Lake 

 group, Punchbowl, Diamond Head, Koko Head, the Kaneohe group and other 

 smaller craters of similar character. During this period tlie tutl' came up 

 through coral reefs, the land as we know it being submerged in the region 

 of eruption. Then followed a long period of decay and the disintegi-ation of 

 the older eruptions and the newer tufiP-eones of sufficient duration to produce 

 soils from them. This period culminated in the discharge of ashes from Tantalus, 

 Punchbowl, Diamond Head, Koko Head and other members of this group of 

 craters, which terminated usually in a more or less extensive shower of vol- 

 canic stones. Dikes were then intruded into crevices, cutting Punchbowl, Dia- 

 mond Head, and the coral reefs at various points, notably at Kaena Point, 

 Kupikipikio and Koko Head. 



Time then elapsed for the accumulation of calcarious talus breccia with 

 soil and vegetation on the lower slope of Diamond Head sufficient to support 

 several species of land shells. Then apparently came the depression of the 

 whole island during which time the ocean encroached on the land above its present 

 level, submerging the low lands about the island. This comparatively brief 

 period left ocean deposits and slight wave markings about the new shore line, 

 which, when the island was again elevated to its present level, was marked by 

 ocean-flooded sand dunes — over which more recent dunes have been piled by 

 the action of the wind. Lastly comes the long periods of disintegration, the 

 formation of surface soil and finally human culture. While geologists may dis- 

 agree, and there is much ground for disagreement, in the interpretation of the 

 records in minor matters, all are agreed in the main points, and freely state 

 that almost inconceivable time has elapsed since the oldest part of Oahu first 

 emerged as a volcanic island. 



Theory op the Formation of the Group. 



Among the various theories that have been advanced in attempts to recon- 

 struct the past history of the group, one of great interest and significance has 

 recently been brought forward, in a very concrete form, by Dr. Henry A. Pilsbry, 

 that has as its basis an exhaustive study of the Hawaiian land shells. ^^ 



He finds this interesting portion of the fauna belonging chiefly to a branch 

 of a very ancient group '- of land moUusks that are distributed on various 

 islands of the Pacific. As there is a marked absence of modern types of land 

 moUusks — save those that have been introduced through commerce — he feels that 

 the peculiar fauna cannot be considered as springing from accidental intro- 

 duction in the group from time to time in the remote past. By analogy the 

 conclusion is arrived at that "the Achatinellidce had already difi:'erentiated as a 

 famih- before the beginning of the Tertiary." But the close relationship of the 



11 Aclintiiiellida: '= Orthiirethra. 



