Dr. Bicketts — On the Cause of the Glacial Period. 577 



from the Pacific where a comparatively slight depression would 

 cause the two oceans to mingle. The Panama Eailway cuts through 

 a ridge which is 299 feet above the sea-level ;* and, near Lake Nicar- 

 agua, the lowest pass is 133£ feet above the sea, 2 whilst the isthmus 

 nowhere attains the height of 1000 feet. 3 Should depression take 

 place so as to submerge these areas, there would be no impediment 

 to the Atlantic Equatorial waters passing into the Pacific Ocean, 

 for the mean height of the former is somewhat greater than that of 

 the latter — that is, it is somewhat banked up by the action of the 

 trade-winds. 



In considering the West Indian Islands as the remains of a sub- 

 merged part of the continent of South and Central America, Mrs. 

 Somerville has given the true explanation of the formation of the 

 Gulf of Mexico; but the depression by which they have been 

 formed has extended to a greater depth than the present. In 

 Jamaica the Tertiary strata are more than 5000 feet thick, 4 and 

 Santiago in San Domingo, situated 2000 feet above the sea, rests on 

 Tertiary strata. 5 The whole valley of the Mississippi to beyond its 

 junction with the Ohio once formed a portion of the Gulf of Mexico, 

 the land having sunk considerably below its present level. 



Former depression has also taken place along the western coast. 

 Professor Newberry observed a sea-beach, containing shells similar 

 to those now existing in the ocean below, at 80 or 90 feet above high- 

 water mark, and also at a still greater elevation ; 6 and the Gulf of 

 California is but a submerged extension of the valley of the Colorado 

 Eiver. 



It is not probable that subsidence could have occurred to so great 

 an extent on both its sides without the same process also affecting 

 the isthmus. 



The present fauna on the different sides of the isthmus affords in- 

 dications of a former intercommunication of the two oceans ; by the 

 identity of species in some instances, by the similarity in others. 

 Mr. Philip P. Carpenter (British Association Eeport, 1856) regards 

 35 species of shells as identical in the two oceans ; 34 species are so 

 nearly allied that they may prove to be identical; and 41 species really 

 separated but by very slight differences only. Professor Wyville 

 Thomson, in "Depths of the Sea," arranges side by side 18 Echino- 

 derms from each sea, " which resemble one another so closely in 

 habit and appearance as to be at first sight hardly distinguishable." 



There have been few, if any, investigations made for the purpose 

 of determining the question, but these evidences are almost conclu- 

 sive that submergence of the isthmus has taken place so as to permit 



1 Admiralty Chart. 



2 The Naturalist in Nicaragua. By Thos. Belt, F.G.S. Page 35. 



3 Tertiary Beds in St. Domingo. By T. S. Heneken Quart. Journ. Geol. 

 Soc., vol. vi. p. 44. 



4 Notice of the Geology of Jamaica. By P. M, Duncan, M.B., Sec. G. S., and 

 G. P. "Wall, F.G.S. Quart Journ. Geol. Soc, vol. xxi. pp. 5 and 6. 



5 On Tertiary Beds in San Domingo. From Notes by T. S. Heneken. Quart. 

 Journ. Geol. Soc, vol. vi. page 39. 



6 Colorado Exploring Expedition. — Geology. By Dr. J. S. Newberry. Page 12. 



