On the Geology of the Key of Sombrero, W. I. 251 



XXXI. — On the Geology of the Key of Sombrero, W. 1. 

 Tlates IV. and V. 



By Alexis A. Julien. 

 Assistant in the School of Mines, Columbia College. 



Read March 12, 1866. 



Sombrero is the most northern Key of the Lesser Antilles, 

 lying at the angle made by this chain with that of the larger 

 West India islands running from east to west. Unlike every 

 other coral-island of modern formation it is intersected by a 

 series of fissures or joints of five distinct systems, the first two 

 running downwards from the surfaces of two of the beds, and 

 the last three commencing in the present denuded surface of 

 the Key. All have been filled with loose deposits subsequently 

 formed upon the surface — the last three by varieties of rock- 

 guano whose characteristics are considerably different. From 

 the great bulk of these latter deposits it is evident that the surface 

 of the island has repeatedly remained above the sea-level during 

 a very great period for the formation of each. It is plain, too, 

 that the area of the island was at first vastly greater than now, 

 and most likely identified with that of the submarine bank of 

 which the Key is now the summit, the area decreasing steadily 

 during the whole history, until worn down to its present small 

 dimensions. These estimates, as to time and area, are corro- 

 borated by the discovery of the fossil remains of land-turtles 

 which have been referred by Prof. Jeffries Wyman to three 

 new extinct and gigantic species similar to those of the Gali- 

 pagos Islands. The Key was uninhabited until the year 1856, 

 since which time it has been occupied by parties engaged in 

 quarrying the guano-deposits, observations on which will be the 

 subject of another paper. 



§ 1. Situation. — According to the most reliable observations 

 the position of its centre is in Lat. 18° 36' K and Long. 63° 27' 

 45" W. It divides the channel between the Yirgin and St. 

 Martin banks (which are submerged to a depth of about a dozen 



