No. 13. — Reports on the Dredging Operations off the West Coast of 

 Central America to the Galapagos, to the West Coast of Mexico, 

 and in the Gxdf of California, in charge of Alexander Agassiz, 

 carried on hy the U. S. Fish Commission Steamer "Albatross," 

 Lieut. Commander Z. L. Tanxek, U. S. N., Commanding. 



[Published by permission of Marshall McDonald, U. S. rish Commissioner.] 



V. 



Ueport upon Rocks collected from the Galapagos Islands. By George P. 



Merrill. 



So far as the present writer is aware, the volcanic roclcs of the 

 Galapagos Islands have been the subject of but little investigation. 

 Darwin in his "Voyage of the Beagle " (pp. 372 et seq.) describes the 

 islands as volcanic, and the character of the material as black basaltic 

 lava with scoria and tuffs. Naturally, his studies did not at that time 

 include an examination of thin sections under the microscope. A more 

 detailed description of the rocks of these islands is given by Dr. F. A. 

 Gooch, in Tscherraak's " Mineralogische Mittheilungen" for 1876 (pp. 133 

 et seq.). x\ccording to the latter, the volcanic materials examined by 

 him are all of a basaltic character, and in part closely correspond to 

 those at present under consideration. Dr. Gooch's specimens were how- 

 ever from the islands of Bindloe, Abington, Hood, and Cliarles, none 

 of which localities are represented in the series received from the Fish 

 Commission. 



A reference to this paper is however of importance. How well the 

 speicmens now examined represent the sources from which they were 

 taken, the present writer has no means of deciding. It is assumed that 

 they are fairly typical. 



VOL. XVI. — NO. 13. 



