226 — Hague and Lddings— Volcanoes of 
reat. variation in the structure and condition of the ground 
mass, which affords a most interesting field for studying the 
gradations between a brown glass base and a_holocrystalline 
groundmass. The porphyritic feldspars in the thin sections 
appear to be wholly plagioclase, no orthoclase having been 
recognized, they are very fresh and present between crossed 
icols a most beautiful zonal structure, and quite irregular 
bands of twining, which with sharply defined straight edges 
vary in length, breadth and numbers, and are formed both after 
the albite and pericline laws. Inclusions of glass are very 
abundant in some crystals and entirely wanting in others. 
When of comparatively large size they are scattered irregularly 
through the erystal, but when very minute they occur in aggre 
gations that sometimes form zones at the margin or center of 
the feldspar. From the size of the symmetrical extinction 
angles observed it would appear that a part of the plagioclase 
is anorthite, but that for the most part the feldspar is labra- 
borite or andesine, and such was found to be the case when the 
feldspars from the hypersthene pumice were analyzed cheml- 
. (See analyses II] and IV, table IL. | 
he Fe, Mg, silicate of this rock is pyroxene, partly hypers- 
thene, partly augite, the hypersthene predominating. In thin 
section the latter is a light brown mineral with a tinge of 
green, which in polarized light shows a strong pleochroism, be- _ 
