EEGION OF THE CAELIN PEAKS. 587 



Silica 62.71 



Alumina 12.10 



Ferric oxide 14.79 



Lime 8.34 



Magnesia 1.31 



Soda 0.73 



Potassa 1.15 



101.13 



which is remarkable, not only for the high percentage of iron, but also for 

 the absence of water and the small proportion of alkalies. 



The trachyte which forms the main body of the hills to the north of 

 Palisade Canon, and the narrow gorge at the lower end of this canon, is a 

 somewhat different rock from that found along the south banks of the river, 

 and apparently of later origin. It is a light-gray, quite porous sanidin- 

 trachyte, noticeable for the large earthy hornblende crystals, which occur 

 in remarkably perfect hexagonal prisms, often as much as one-sixteenth of 

 an inch in diameter. Casts of these crj^stals are found on the weathered 

 surfaces of the rock, retaining perfectly the form of the original crystal. In 

 general, the hornblendes are very much decomposed, and have imparted a 

 somewhat reddish, rusty stain to the groundmass. Under the microscope, 

 the latter is seen to contain some glassy base and also a little tridymite. 

 The decomposition of the hornblendes is also very peculiar, and has often 

 proceeded so far that the interior core of the crystals has been removed, and 

 in part replaced by groundmass-material. In one specimen of this trachyte 

 from the south bank of the river were found a few yellowish-brown crystals 

 of augite. 



Region of the Carlin Peaks. — North of Palisade Canon, for a dis- 

 tance of about 20 miles, the surface of the Cortez Range is principally cov- 

 ered by extensive flows of rhyolite. A few obscure outcrops of sediment- 

 ary rocks show a pre-existing line of elevation, similar to that south of the 

 river, whose form was probably determined prior to the volcanic period. 

 These outcrops are, however, too isolated and indistinct to give any clue as 

 to their structure, or any definite idea of the age of the rocks. The princi- 



