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



Often the mashed rock is somewhat lighter in color than the 

 massive varieties. 



The microscope reveals the fragments to be of at least four 

 kinds : (1) fragments of an andesitic flow rock, with a pilo- 

 taxitic gronndmass of a flnidal texture ; (2) fragments of ande- 

 sitic rock without trachytic arrangement of the feldspar laths ; 

 (3) fragments of an amygdaloidal andesite; and (4) fragments 

 of an acid rock, probably a tuff. 



The wide distribution of the type of rock just described, 

 and its diversity of contained fragments, suggests the great 

 complexity of the volcanic period during which it was formed. 

 There were undoubtedly many alternations between outbreaks 

 of acid and comparatively basic magmas. 



Andesite. — Andesite is of limited occurrence within the dis- 

 trict. It forms several narrow strips and lenses of small areal 

 extent, which represent the remnants of old flows. The rock 

 is massive and mostly porphyritic. One occurrence is amygda- 

 loidal and dotted with small rounded and oval areas of green- 

 ish material, representing the vesicles of a surface lava subse- 

 quently filled with infiltrated material. The outcrops are not 

 large, but are fairly abundant, and are usually rounded. 



The porphyritic andesite is a medium to fine-grained rock, 

 varying in color from a grayish green, or epidote green mottled 

 with blue, to a dark bluish purple. All variations contain 

 small green specks and masses of epidote. Except in the 

 densest specimens, the rock is easily seen to be porphyritic ; 

 the phenocrysts are feldspar laths of variable abundance. 



The amygdaloidal phase is an epidote-green rock, abundantly 

 dotted with rounded and elliptical amygdules, varying in 

 diameter up to 4 mm . These are filled with material of a darker 

 green color, which in most cases is epidote with chlorite, but 

 sometimes in part calcite. Near the surface the material 

 filling the cavities has weathered out, giving to the rock a 

 honeycombed appearance. 



The analysis of the rock, made by the writer, is : 



Si0 2 AI0O3 Fe 2 3 FeO MgO CaO Na 2 K 2 H 2 C0 2 Total 

 66-28 10 62 6-41 2"11 1-15 3-17 6-09 1-73 0-61 1-47=99-64 



From this, its position in the quantitative system has been 

 calculated to be: Class II, dosalane ; order 4, quardofelic ; 

 rang 1, peralkalic ; subrang 4, dosodic. The rock corresponds 

 to jpantellerose. 



The rock on the whole is a very alkalic andesite. Certain 

 phases have a strong trachy-andesitic stamp. . 



Dike rocks. GaVbro. — Gabbro occurs widely and abund- 

 antly distributed in the form of dikes, trending in a northeast- 

 southwest direction. In size these vary from one-eighth mile 



