GEOLOGY AND TOPOGRAPHY OF HAWAII. 123 



genera of the sub-family Amastriiue and the even closer rclatioiislii]) of tlie 

 genera of the related sub-family AchatineUimt "indicate a sudden i-ejuvene.scence 

 of the old stock in comparatively modern ^^ time." A study of the species, 

 varieties and forms extant show that everywhere intense local differentiation 

 is still in progress. 



Dr. Pilsbry concludes that "the logical geographic l)()uiidaries of most of the 

 species of AchaiineUida' give excellent ground for the belief that the ju'eseiit 

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

 locomotion and that unusual or so-called accidental carriage, as hy ])irds, 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 frcmi 

 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 Midway 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 distril)ution 

 of the land forms had taken place. 



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

 shells studied our authority deduces the following sequence of events, the be- 

 ginning of which is placed probal)ly in the Mesozoic, possibly in Eocene time. 



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

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

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

 canism which gave their present topography to the islands and ]>robMl)ly d;ited 

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



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

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

 75, fig. 2.) 



III. "Northern Hawaii was next isolated hy formaticm of the AU'nuihala 

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

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



lY. "In the eastern end of this Oahu-^Iaui island arose certain genera,^* 

 while another ])eeuliar genera ^^ was evolved in tlie Avest from undoubted nn- 

 cesteral stock. 



Y. "The Oahuan and the ^Folokai-Lanai-^lauiau areas were sundered by 

 subsidence of tlie Kaiwi Channel." (Plate 75, fiu. 4. i On Oahu the niollusean 

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

 islands, the western or Waianae and the eastern or Koolau area. Tii each area 

 certain genera were differentiated, but latei'. in the later Pliocene or IMeistoeene 



13 Tertiary. i-* LamiiicllK. i» Pterodiscus. 



