CLASSIFICATION OF ARKOSE DEPOSITS 



437 



show a slightly greater degree of rounding. It is notable that the 

 predominant feldspar of the arkose, microcline, does not occur in 

 the underlying terrane. Where the arkose rests directly on the 

 mountainous topography of the underlying terrane, the arkose 

 basally becomes a coarse breccia. One of the most striking features 

 of this formation, with the exception of this basal portion, is its 

 uniformity throughout its great thickness. 



To this type of arkose 

 deposit should be referred 

 the following: 



The Torridonian Arkose, 

 pre-Cambrian of Scotland. 



Arkose of the Sparagmite 

 formation, pre-Cambrian of 

 Scandinavia 



Lower Old Red Sand- 

 stone, Scotland (in part) 



Paysaten Arkose, Creta- 

 ceous, British Columbia- 

 Washington 



b) Marine: If marine 

 conditions prevail adja- 

 cent to a granitic terrane 

 in a desert region, marine 

 arkose may form, having 

 in part the characteristics 

 of an eolian arkose. Some of the constituent grains in this case 

 should show the rounded outlines of eolian sand grains. The 

 deposits as a whole, however, should show the structure and 

 stratification of marine sediments. To this type of deposit should 

 be referred the arkose that is now forming along the east shore of 

 the Gulf of California. 



2. High -altitude deposits. — Local deposits, of small size and 

 extent. The conditions of high altitude, according to Oldham and 

 others, are peculiarly favorable to disintegration. Erosion of the 

 disintegrated material takes place rapidly, with rapid deposition 

 of it in many cases as arkose in local catchment basins of the 

 intra-mountain valleys. As such a region is subject to general 



Fig. 3. — Photomicrograph of Terridonian 

 arkose, Applecross, Scotland, showing the lack of 

 matrix in a desert arkose. The rounding of the 

 grains is rather obscured by secondary growth of 

 the grains. Magnification, 15 diameters. 



