458 A. F. BUDDINGTON 



Near Manuals a plug of plagioclase basalt about 400 by 450 feet, 

 with columnar jointing perpendicular to the surface and with dikes 

 and apophyses radiating from it into the adjoining rocks, was found 

 intruded through basaltic and rhyolitic breccias. A rhyolite neck 

 about fifty feet in diameter and circular in shape, nearly every foot 

 of the contact of which could be seen, and a neck of basic tuff with 

 fragments up to three feet in diameter of the underlying rocks, 

 through which it was drilled, and with radiating dikes of tuff, were 

 also found. The largest neck forms a conspicuous hill about three 

 miles south of Holyrood and seems to have been ofie of the main 

 centers of volcanic activity in this region. Here we find a great 

 elliptical stock of rhyolite porphyry about a mile long and half a 

 mile wide, while in the surrounding country occur exceptionally 

 coarse rhyolite breccias, with many blocks up to two feet in diam- 

 eter, interbedded with rhyolite flows and tuffs. A chemical analysis 

 of this rock, given below, shows it to be very similar to the rhyohte 

 flows at Manuels. 



TABLE I 



Chemical Analysis or Rhyolite Porphyry from Volcanic 

 Neck South of Holyrood 



SiO. 75 



AI2O3 13 



FeA I 



FeO 



MgO.. 



CaO 



Na.0 3 



85 K3O 4-47 



63 H.0+ 35 



82 H,0- 05 



32 MnO 05 



05 



38 Total. ......99.71 



34 



The rhyolite flows are quite variable in color, ranging through 

 reddish, purplish, and greenish grays. They are usually aphanitic 

 in texture, and dense, chertlike in character, sometimes felsitic, with 

 a finely rough feel. They may be homogeneous and massive, or 

 may be marked by beautiful, well-developed, and wefl-preserved 

 banded textures, and are very frequently spherulitic. The spheru- 

 lites vary from microspherulites up to those as large as a man's head, 

 or even occasionally attain a diameter of two feet (Fig. 4), but in 

 general they are of medium size (Fig. 5). The flows may also be 

 characterized by a platy jointing more or less parallel to the flowage 

 planes, or by flow breccia, eutaxitic (Fig. 6), or perlitic structures. 



