Vol. VII] DICKERSON— CLIMATE AND OLIGOCENE FAUNAS 163 



closely related to the Molopophorus lincolnensis zone than to 

 the upper, the Acila gettysburgensis zone. 



Weaver* described the faunal characters of this last zone 

 as follows : "The most characteristic species of the Acila 

 gettysburgensis zone are Acila gettysburgettsis Reagan, Mac- 

 rocallista vespertina (Conrad), Marcia oregonensis (Conrad), 

 Modiolus rectus DaW, Panope generosujn (Gould), Phacoides 

 acutilineatus (Conrad), Spisula albaria (Conrad), Solemya 

 ventricosta Conrad, Tellina oregonensis Conrad, Thracia trap- 

 ezoidea Conrad, Thyasira bisecta (Conrad), Crepidula prce- 

 rupta Conrad, Eudolium petrosum (Conrad), Miopleiona 

 indurata (Conrad), Turcicula washingtoniana Dall and Tur- 

 rit ella blakeleyensis Weaver. Such species as Acila gettys- 

 burgensis Reagan, Solemya ventricosta Conrad, Eudolium 

 petrosum (Conrad) and Turcicula ivashingtoniana Dall ap- 

 pear for the first time in this zone. They are always the most 

 common species met with and are entirely absent from the 

 Turritella porterensis and Molopophorus lincolnensis zones." 



Arnold and Hannibal's list from their Seattle horizon is 

 essentially the same. The disappearance of many tropical 

 genera, the introduction of several temperate genera, are 

 noteworthy temperate faunal conditions. That the Turritella 

 porterensis zone was tropical or semi-tropical is well attested by 

 the occurrence of the reef-building coral, Dendrophyllia hanni- 

 bali Nomland and other tropical genera. All the known facts 

 considered, we may then conclude that the Molopophorus lin- 

 colnensis and Turritella porterensis zones were deposited under 

 tropical or subtropical conditions and the Acila gettysburgensis 

 zone, under temperate conditions somewhat warmer than those 

 of today in that latitude. What was the reason for this faunal 

 change? May we invoke the great god Diastrophism to aid 

 us in explanation? Probably a depression in the vicinity of 

 the Bering region of Alaska occurred at the beginning of the 

 deposition of the Acila gettysburgensis zone and cold boreal 

 waters of the Arctic sea brought with them a boreal fauna 

 some of whose members managed to establish themselves in 

 Washington, in some cases even crowding out the native 

 species. The known history of Oligocene vertebrates gives 



* Weaver, C. E., The Post-Eocene Formations of Western Washington, Proc. Cal. 

 Acad. Sci., Fourth Series, vol. 6, pp. 30-32, 1916. 



