Pogue, Jr. — Geology and Structure of Volcanic Rocks. 227 



The occurrence probably represents the remnant of an old 

 surface flow, of slightly more basic nature than the flows of 

 rhvolite. Its rounded contour on the map is due to the fact 

 that the observer is not looking down upon the upturned edge 

 of the flow, but more nearly upon its horizontal surface. 

 Since there is no direct evidence of flow structure, the mass 

 may represent an old volcanic neck or conduit, or perhaps an 

 intrusion or sill which never reached the original surface. 



The basic volcanic rocks. Andesitic fine tuff. — The 

 andesitic fine tuff represents consolidated dust and ashes from 

 explosive eruptions of more basic nature than those which 

 gave rise to the acid series of rocks. Fragments are almost 

 entirely wanting and are never visible to the unaided eye. 

 Upon addition of these, the rock passes into the andesitic 

 tuff and breccia, with which it is closely associated. In no 

 place is its areal distribution of sufficient extent to show upon 

 the geologic map. Its separate description is warranted from 

 its analogy to the acid line tuff. 



The rock is dense and somewhat less siliceous than its acid 

 analogue. In color it is either greenish or green mottled with 

 purple. Outcrops are small, rounded, of grayish-green 

 exterior, and sandpaper-like surface. 



Andesitic coarse tuff and breccia. — The andesitic coarse 

 tuff, composed of grounclmass, phenocrysts, and a subordinate 

 number of fragments ; and the andesitic breccia, with pre- 

 dominant and larger fragments, are described together. Their 

 intimate association makes it impossible to separately map the 

 two. 



These rocks form long, narrow strips and broader lenses of 

 important areal extent, alternating with the areas of slate and 

 the acid series of volcanic rocks. They range all the way 

 from massive varieties, made up almost wholly of green frag- 

 ments, to greenstone schists, which in themselves contain little 

 evidence of their fragmental nature. The outcrops are abund- 

 ant and prominent ; when massive, they are low and well 

 rounded ; with increasing degree of schistosity, they become 

 elongated and narrow, and resemble much in shape great 

 inverted wedges. Only a few extremely schistose occurrences 

 fail to have a humpy weathered surface, which reveals the 

 fragmental nature of the rock, even when this feature is not 

 observed on fresh fracture. 



The massive rock is heavy, tough, dark green, and composed 

 almost entirely of dark green fragments up to three-quarters of 

 an inch and larger in diameter. Dark green material, containing 

 feldspar phenocrysts, fills the spaces between the fragments. 

 More schistose varieties appear less fragmental ; the fragments 

 have been converted into areas of greenish secondary minerals. 



