IGNEOUS FORMATIONS. 313 



mountain, tins tuff is very well .shown. Beds 5 and 6 of the section of 

 the former locality exhibit the rock in its purest condition, and the speci- 

 mens particularly described come from these beds. In the series included 

 under (3) of the same section there are some lasers agreeing nearly with 

 the same description, but even the purest of these contain enough rounded 

 grains of different character to cause their classification rather with the 

 transition strata. 



Plant remains are sparingly present in the tuff, and show the sedi- 

 mentary character in cases where there might he some doubt from mere 

 megascopical examination whether certain rounded masses were massive 

 andesite or tuff of similar composition. 



Description. — The pure aiidt'sitic t uff pa rti< -ula I'l v mentioned above is very 

 light gray or straw-colored, finely, evenly grained, and rather friable as 

 exposed in the outcrops. A few black ore grains and some few particles 

 of biotite, hornblende, or green augite may be distinguished in the light- 

 colored mass, which consists chiefly of glassy feldspar fragments. 



The sedimentary character of this tuff is usually plain, through the 

 more or less banded structure produced by the varying amounts of darker 

 constituents in different layers. Leaves and stems of plants are scattered 

 through the mass, though never very abundantly. 



As was mentioned in describing the section at the locality in question, 

 the lower part of this tuff bed resembles a conglomerate in that there are 

 pebble-like masses lying in a matrix of somewhat different appearance. 

 The rounded masses are pebbles of tuff, and the matrix in which they lie 

 is finer-grained material of the same kind. These pebbles represent a 

 layer of tuff broken up during a turbulent period and redeposited partly as 

 pebbles and partly as sand. 



Microscopical study of this tuff shows it to lie predominantly made up 

 of plagioclase, with some augite, hornblende, biotite, and magnetite. The 

 particles are sharply angular and fresh, and are identical in character. 

 the presence of inclusions of glass, apatite, etc., with the constituents of 

 andesites represented in the pebbles of some adjacent conglomerates. In 

 this special layer the constituents are present in about the normal propor- 

 tion for a massive andesite. No foreign matter, such as quartz or feldspar 



