chap. xni. Crystalline Rocks of, 443 



seen either by M. d'Orbigny or Professor E. Forbes, as, 

 since my return to England, the specimens have been 

 lost. The great clay-slate formation of Tierra del Fnego 

 being cretaceous, is certainly a very interesting fact, — 

 whether we consider the appearance of the country, 

 which, without the evidence afforded by the fossils, 

 would form the analogy of most known districts, pro- 

 bably have been considered as belonging to the Palaeo- 

 zoic series, — or whether we view it as showing that the 

 age of this terminal portion of the great axis of South 

 America, is the same (as will hereafter be seen) with 

 the Cordillera of Chile and Peru. 



The clay-slate in many parts of Tierra del Fuego, 

 is broken by dikes l and by great masses of greenstone, 

 often highly hornblendic : almost all the small islets 

 within the clay-slate districts are thus composed. The 

 slate near the dikes generally becomes paler-coloured, 

 harder, less fissile, of a feldspathic nature, and passes 

 into a porphyry or greenstone : in one case, however, 

 it became more fissile, of a red colour, and contained 

 minute scales of mica, which were absent in the un- 

 altered rock. On the east side of Ponsonby Sound, 

 some dikes composed of a pale sonorous feldspathic 

 rock, porphyritic with a little feldspar, were remarkable 

 from their number, — there being within the space of a 

 mile at least one hundred, — from their nearly equalling 

 in bulk the intermediate slate, — and more especially 

 from the excessive fineness (like the finest inlaid car- 

 pentry) and perfect parallelism of their junctions with 

 the almost vertical laminae of clay-slate. I was unable 

 to persuade myself that these great parallel masses had 

 been injected, until I found one dike which abruptly 



1 In a greenstone-dike in the Magdalen Channel, the feldspar 

 cleaved with the angle of albite. This dike was crossed, as well as 

 the surrounding slate, by a large vein of quartz, a circumstance of 

 unusual occurrence. 



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