606 TIERRA DEL FUEGO. 
Decomposition.—T he igneous rocks decompose rapidly on exposure, 
becoming bleached and losing their crystals of feldspar. The removal 
of these crystals gives a cellular and somewhat scoriaceous appearance 
to the external surface of the rock. Some varieties, especially the 
trachytes, become white externally to a depth of an inch, and resem- 
ble a pumice. Owing to this effect, the summit of a hill composed of 
a dark-coloured rock often presents a white chalky appearance. 
The minerals which have been described as constituting nodules 
and veins in the trap and amygdaloid, bear evidence of having been 
formed subsequently to the ejection of the rock, and fill cavities left 
when the bed cooled. Without presenting the various arguments on 
this point, which have already been published by the writer in the 
American Journal of Science, volume xlix. page 49, a few particulars 
may be alluded to. 
The globules of lime in the amygdaloid described, are covered with 
a thin incrustation of a greenish-black earth, and appear externally to 
be black shining globules. This incrustation is the same chloritic 
material so common in amygdaloids, and which appears to be the first 
deposit from the infiltrating waters. The stilbite veins occur not 
only in the greenstone, but also in the conglomerates in the vicinity 
of the veins. 
With regard to the chalcedony, there is some reason to suppose it to 
have been deposited immediately upon the cooling of the rock, since 
its veins are very irregular in size, and twist around in every direction. 
