300 DR. J. W. GREGORY ON THE PALEONTOLOGY [Aug. 1895, 



direct evidence of the geology of Central America ; 2ndly, the 

 indirect evidence yielded hy the faunas of the two oceans. The 

 former rests in the main on a collection made by Dr. G. A Maack l 

 during his survey of the Isthmus of Darien. It has been stated 

 that he found numerous marine Pleistocene fossils on the very 

 summit of the watershed. This does not, however, appear in his 

 own account. According to this the summit of the ridge, which is 

 crossed by the Napipi route at the height of 612 feet, is of old igneous 

 or schistose rocks ; the marine deposits on the lower slopes are 

 Pliocene, and those at higher elevations early Tertiary. He assigns 

 the connexion between the two seas in one passage to the Pliocene, 

 and in another to the early Tertiary. Mr. Warren Upham 

 first quoted Maack as proving the submergence of the isthmus 

 to the summit of the main pass, at the height of 763 feet, in 

 Pliocene and possibly Pleistocene times. 2 In a later note on 

 the subject he claimed, from the work of the same observer, that 

 a submergence of the whole isthmus had almost certainly taken 

 place in Pleistocene time. 2 Maack's collection was said to be in 

 Washington, where I tried in vain to find it. 



It was, however, seen by Gabb in Costa Rica in 1873, and 

 he identified in it many Miocene species. Unfortunately, it is 

 impossible now to tell to what subdivision of the Miocene the fossils 

 belonged. Gabb has himself collected Miocene fossils from the 

 summit of the central ridge of Costa Pica ; he describes the deposits 

 of this age as forming the whole of the Atlantic slope, and has little 

 doubt that they extend thence uninterruptedly to the Pacific, though 

 he did not trace them. 3 But here again he gives no information as 

 to what part of the Miocene the fossils indicate. 



The abstract of Dr. Crawford's paper ' On the Geology of Nica- 

 ragua,' 4 issued in 1891, also gives no support to the view of the 

 occurrence of marine Pleistocene beds above the watershed. 



We must, therefore, conclude that, though there is no doubt that 

 marine Pleistocene deposits occur at the height of 150 feet up 

 the Atlantic slope, 5 there is not the slightest reliable geological 

 evidence of any submergence of the isthmus in any period later 

 than the Miocene of Gabb, and this Miocene may be Oligocene. 



The Relations of the Caribbean and Panamaic Faunas. — The 

 recent date of that event, however, has also been affirmed from a 

 comparison of the marine faunas on either side of the isthmus. 



1 G. A. Maack, ' Reports of Exploration for a Ship Canal, Isthmus of 

 Darien/ Public Doc. Navy Dept. no. 1144, vol. v. pp. 155-175; ' The Secret of 

 the Strait,' Harper's New' Monthly Mag. vol. xlvii. (1873) p. 812. 



2 Warren Upham, ' On the Cause of the Glacial Period,' Amer. G-eol. vol. vi. 

 (1890) p. 329 ; ' Pleistocene Submergence of the Isthmus of Panama,' ibid. 

 p. 396. 



3 W. M. Gabb, ' Notes on Costa Rica Geology,' Amer. Journ. Sci. ser. 3, 

 vol. ix. (1875) pp. 198-204. 



4 Rep. Brit. Assoc. 1890 (1891) pp. 812, 813. 



5 W. M. Gabb, ' Miocene Fossils in Santo Domingo,' Proc. Amer. Phil. Soc. 

 toI. xii. (1873) p. 572. 



