Rocky Mountains, 403 



ning parallel to each other, and at right angles with the hori- 

 zon. Following the water courses, which are sunk a consi- 

 derable distance below the surface, the line of separailoa 

 from the sandstone, on which the greenstone rests, at length 

 becomes visible. 



2d. — Amygdaloid. We apply this name to a porous or ves- 

 sicular rock of a very dark gray, greenish, or black colour, usu- 

 ally found near the greenstone, but sometimes in connexion 

 with the sandstone. In its ultimate composition it resembles 

 greenstone, but we have never seen in it such lar^e frag*' 

 ments of feldspar and scales of mica as are observed in that 

 rock- The amy gdaloidal cavities, with which it is every where 

 penvitrated, are of various sizes; some of them appear like 

 bubbles, which had been formed in a semi-fluid mass, and 

 afterwards lengthened and variously distorted by the mo- 

 tions of the contiguous matter. Near the surface they con- 

 tain a soft, white, or yellowish-white substance, very differ- 

 ent from the rock itself, usually a pulverulent carbonate 

 of lime. This gives the recent surface a mottled appear- 

 ance. In surfaces which have been for some time exposed to 

 the air this soft substance has been removed, and the pores 

 and vessicles are empty. 



Amygdaloid does not appear to occupy any very great ex- 

 tent of the country near the Rocky Monntains; we have not 

 met with it imbedded in, or surmounted by any other rocks. 

 It forms conic hills like the greenstone, which sometimes oc- 

 cur in deep water-worn vallies, bounded on both sides by per- 

 pendicular walls of sandstone. It is likewise seen in the 

 high plains, sometimes in the form of narrow and crooked 

 ridges, apparently following what were formerly the beds of 

 small brooks. Some very high and sharp conic hills 

 were visible to the westward, but at a great distance. Two 

 of this kind, which stand near each other, and seem to be de- 

 tached from the primitive mountains, are called the Spanish 

 peaks, and at the end of July snow was still to be seen on 

 them. 



Where either of the two rocks last mentioned occur it is not 

 uncommon to find detached masses of a stone somewhat re- 

 sembling the pumice stone of commerce. It is usually of a 

 faint red, or yellowish-white colour, but sometimes brown, 

 or nearly black. It feels less harsh than the pumice stone 

 used in the arts, and seems to consist in a great degree of 

 clay. It appears to be entirely similar to the substance 

 brought down the Missouri by the annual floods, and by ma- 



