GEOLOGY AND TOPOGRAPHY OF HAWAII. 123 



genera of the sub-family Ai)tastriiue and the even closer i-elatiouship of the 

 genera of the related snb-family Achat in ell iiue "indicate a sudden rejuvenescence 

 of the old stock in comi^aratively modern ^-^ time." A study of the species, 

 varieties and forms extant show that everj-vvhere intense local differentiation 

 is still in progress. 



Dr. Pilsbry concludes that "the logical geographic boundaries of most of the 

 species of Achatinellidce give excellent ground for the belief that the present 

 distribution of all the larger species has been attained by their own means of 

 locomotion and that unusual or so-called accidental carriage, as by birds, drift- 

 ing trees, etc., has been so rare as to be negligible. No evidence whatever of 

 such carriage is known to me. ' ' 



After exhausting the possibilities of accidental introduction of species from 

 island to island, the conclusion follows that all of the important islands must 

 have been, at one time, connected by land, and that distribution of the an- 

 cestral forms of land shells from Kauai to Hawaii was effected at that time. 



As the Hawaiian chain, from Ocean and ]\Iidway Islands to Hawaii, a 

 distance of 1,700 miles, rests on a submarine ridge, the greatest depth between 

 the islands being less than 3,000 fathoms, the distribution and subsequent isola- 

 tion of the forms on the islands appear to be in accord with the theory of sub- 

 sidence of the ridge supporting the entire archipelago after wide distribution 

 of tlie land forms had taken place. 



From the affinities and the geographic relations of the several groups of land 

 shells studied our authoi'ity deduces the following sequence of events, the be- 

 ginning of which is placed probably in the Mesozoic, possibly in Eocene time. 



I. "The Hawaiian area from northern Hawaii to and probably far beyond 

 Kauai formed one large island which was inhabited by the primitive Amastrince. 

 This pan-Hawaiian land, whatever its structure, preceded the era of vul- 

 canism which gave their present topography to the islands and probably dated 

 from the Paleozoic." (Plate 75, fig. 1.) 



II. "Volcanic activity built np the older masses, subsidence following, 

 Kauai being the first island dismembered from the pan-Hawaiian area." (Plate 

 75, fig. 2.) 



III. "Northern Hawaii was next isolated by formation of the Alcuiuliala 

 Channel, leaving the large intermediate island, which included the present islands 

 of Oahu, Molokai, Lanai, and Maui." (Plate 75, fig. 3.) 



IV. "In the eastern end of this Oahu-Maui island arose cei'tain genera,^* 

 while another peculiar genera ^^ was evolved in the west from undoubted an- 

 cesteral stock. 



V. "The Oahuan and the Molokai-Lanai-Mauian areas were sundered by 

 subsidence of the Kaiwi Channel." (Plate 75, fig. 4.) On Oahu the molluscan 

 fauna bears out the generally accepted theory of two centers, probably two 

 i.slands, the western or Waianae and the eastern or Koolau area. In each area 

 certain genera were differentiated, l)ut later, in the later Pliocene or Pleistocene 



