3- Additional Comments on the Geology of the 

 Juan Fernandez Islands. 



By 



PERCY OUENSEL. 



Contents. 



Page 



Introduction ^^ 



Main Geological Features 40 



Petrology of the Volcanic Formations 44 



Masatierra 4^ 



Masafuera 56 



Regional Relations. 



Tectonic Connections -74 



Petrographic Connections 77 



General Conclusions 79 



Acknowledgements 82 



Tables of Analyses 83 



Bibliography 86 



Introduction. 



The Juan Fernandez Islands consist of Masatierra, Masafuera and Santa 

 Clara. In many publications the name Juan Fernandez has been used to denote 

 Masatierra only, the other islands then being indicated by the names as above. 



The largest" island, Masatierra, situated 660 km from and nearly due west 

 of Valparaiso, measures 95 square km. Masafuera, 170 km further westward, is 

 64 square km. The small island Santa Clara, close to Masatierra, is only 5 

 square km in area. 



During more or less casual visits to Juan Fernandez stray observations have 

 been recorded on geological features of the islands. In all cases they refer to 

 Masatierra. It may be of interest in this connection to give a summary recount 

 thereof. 



The first samples of volcanic rocks of Masatierra were, as far as known, 

 collected by Lord CocilCRANE in January 1823. He was returning to Europe 

 after 5 years service as admiral in Chilean service during the war of independ- 

 ence. The ship at his disposal, 'Colonel Allen', touched at Masatierra for two 

 days. Mrs Maria Graham, a passenger on board, has in her diary given the 

 following details, recorded by Thomas Sutcliffe in his book Cmsoniana: "Lord 



3 — 516795 The Nat. Hist, of Juan Fernandez and Easter Isl. Vol. I. 



