828 DESCEIPTIVE GEOLOGY. 



noticeable by a large distribution of decomposed earthy forms Upon 

 other outcropping masses of audesite, on the hill-tops, Avere found numer- 

 ous sheets of hyalite, existing as an incrustation, from one-sixteenth to 

 one-eighth of an inch in thickness, and following the irregularities of the 

 surface. The appearance of these hyalites, as well as of those found 

 incrusting the trachytic and basaltic rocks of the neighborhood, would seem 

 to indicate a formation by secretion upon the present rock surfaces. 



Directly north of the river, and lying opposite to the above-described 

 andesites, is a large body of sanidin-trachyte, forming a compact heavy 

 structure, of a chocolate-red color, and a distinctly bedded occurrence. 

 Back from the river, about 2 miles, are found certain outcrops having the 

 fine parallel laminae characteristic of the rhyolite group, but otherwise 

 with the habit of trachyte. The rock carries only 56.74 per cent, of silica. 



About 2 miles below the entrance of the canon, and occupying two 

 ravines which open north and south on both sides of the river, is found an 

 exposure of an older rock, of a prevailing dark-purple color, probably a 

 porphyry, in which are plentifully scattered the decomposed crystals of tri- 

 clinic feldspar. These feldspars are frequently decomposed and replaced 

 by calcite ; in others, they exist as a calcareous earthy clay. Here and 

 there are green masses, probably delessite. The entire groundmass of the 

 rock has suffered such chemical decomposition that it is impossible to detect, 

 without the aid of the microscope, whether it is hornblendic or augitic in. 

 origin. The prevalence of calcite and delessite, and the mode of decom- 

 position, would refer it to the rock classed as melaphyr, found in Berkshire 

 Canon; but large, well-characterized feldspars in the groundmass give a 

 preponderance of evidence of its being more nearly related to the porphyries. 

 In a railroad cut on the north side of the river, there is an excellent expos- 

 ure of this rock, where it is seen to be divided into rough, rectangular prisms 

 by vertical and horizontal planes, so that the forms of rude steps are quite 

 noticeable. 



Directly east of the andesite and trachyte masses, the canon for 8 miles 

 down the river, on both flanks, is made up exclusively of basaltic rocks. 

 On the south side of the canon, descending in some places quite to the river- 

 bank, the basalt occurs in long flows, having its origin on the high ridges. 



