3S0 L. V. Pirsson — Note on some Volcanic Rocks 



Art. XLII. — Note on some Volcanic Hocks from Gougtis 

 Island, South Atlantic ; by L. V. Pirsson. 



The rocks which are the subject of this note were gathered 

 as beach pebbles from the shore of Gough's Island, South 

 Atlantic Ocean, lat. 40° 20' S., Ion. 9° 44' W , by the captain of 

 a whaling vessel from JSTew London, Conn., and they came 

 into possession of the writer through the kindness of Prof. 

 C. E. Beecher of the Peabody Museum in New Haven. 



While in general the mere petrographical description of 

 rocks disconnected from their geological occurrence and rela- 

 tionships has little value or interest, the fact that nothing so 

 far as the writer can learn has ever been published on this 

 remote locality, and also that the material developed several 

 points of interest are the reasons for the publication of this 

 note. 



Gough's Island was discovered in the sixteenth century by 

 Goncalo Alvarez, and later by Gough, an English navigator, 

 after whom it is generally called. The island seems to be 

 little known and a search through the literature has given 

 only the information that it is a craggy mass rising to a height 

 of 4350 feet and about eighteen miles around, of a generally 

 agreeable climate, with streams which present several fine cas- 

 cades and several valleys green with turf and bushes.* Tris- 

 tam da Cunha, the group of islands lying 240 miles N.W., are 

 of volcanic origin, as others of the detached oceanic islands of 

 the South Atlantic, and from what has been given and the 

 evidence afforded by the present investigation it is probable 

 that the island represents a volcanic cone. 



The material consisted of a small number of rounded peb- 

 bles, all of which represent very fresh volcanic rocks or tuffs 

 which may be referred either to trachytes or basalts. 



Basalt. 



This is represented by two varieties, one of which is a rock 

 of a dark gray color thickly dotted with black phenocrysts of 

 augite, yellow olivines and white feldspars of the microtine 

 habit The groundmass is dense and cannot be resolved by 

 the eye alone. The augites are the largest phenocrysts, attain- 

 ing at times a length of 5 mm ; the feldspars and olivines are 

 much smaller. In thin section under the microscope the rock is 

 seen to be beautifully fresh and unaltered and the usual minerals 

 to be present, viz : iron ore, apatite, olivine, augite and plagio- 



*Cf. Earth and its Inhabitants; Africa, vol. Hi, pp. 97. New York, 1888. 

 Appleton & Co. 



