266 TiERRA DEL FUEGO. June, 1834. 



long on the top of the mountain. Our descent was not quite 

 so laborious as our ascent ; for the weight of the body forced 

 a passage, and all the slips and falls were in the right di- 

 rection. 



Captain King has given a sketch of the geology of Tierra 

 del Fuego, to which I have httle to add. A great forma- 

 tion of clay-slate, rarely containing organic remains, but 

 sometimes presenting casts of a kind of ammonite, is fronted 

 on the east side by plains belongin g probably to two tertiary 

 epochs. On the west coast, a prolongation of the grand 

 crevice of the Andes, from which so much heat has escaped 

 from the interior of the globe, has metamorphosed the slate. 

 There is, however, a double hne, the structure of which I do 

 not quite comprehend. The interior one consists of granite 

 and mica slate ; the exterior one (perhaps more modern), of 

 greenstone, porphyritic and other curious trappean rocks. 

 Almost every one at first thinks that this country owes its 

 grand name of " the Land of Fire," to the number of its 

 volcanoes. Such, however, is not the case : I did not see 

 even a pebble of any volcanic rock, except in WoUaston 

 Island, where some rounded masses of scoriEe were em- 

 bedded in a conglomerate of no modern date. In a geo- 

 logical point of view this circumstance allows us to consider 

 the grand linear train of ancient and modern volcanoes, 

 which fall on parallel fissures in the Andes, as extending 

 from lat. 55° 40' south to 60° north, a distance httle less 

 than seven thousand geographical miles. 



Perhaps the most curious feature in the geology of this 

 country, is the extent to which the land is intersected by 

 arms of the sea. These channels, as Captain King remarks, 

 are irregular and dotted with islands, where the granitic and 

 trappean rocks occur, but in the clay-slate formation are so 

 straight, that in one instance " a parallel ruler placed on the 

 map upon the projecting points of the south shore, extended 

 across, also touched the headlands of the opposite coast.^' 



I have heard Captain FitzRoy remark, that on entering 

 any of these channels from the outer coast, it is always ne- 



