National Geographic : 1892 Mar 21
H. F. Reid-Studies of Muir Glacier. in southern Greece.* Under the microscope, however, the simi larity is seen to be less close. The groundmass of the Alaskan specimen is much finer grained and more altered. It consists of hypidiomorphic laths of plagioclase, magnetite, and secondary calcite. The large pale green crystals of porphyritic feldspar are very much the same in both rocks. Number 29 is a rock somewhat like that last described, but whose porphyritic crystals are neither so pronounced nor so abundant. Its groundmass is a network of panidiomorphic plagioclase laths connected by a mesostasis which was probably once a glass, but which now is a brown, extremely fine-grained, but brightly polarizing mass carrying chlorite and secondary amphibole. Magnetite is also abundant. Number 46 is probably also classed as an augite-porphyrite. It is macroscopically a green aphanitic rock in which no por phyritic crystals are visible to the unaided eye. Under the microscope it proves to be a panidiomorphic aggregate of plagio clase and a pale gray pyroxene connected by a green interstitial serpentinous mass, which may represent an original glassy base. The feldspar and pyroxene of this rock are both quite fresh. Number 36 may be either an augite-porphyrite or an augite andesite. It is full of zonally banded phenocrysts of plagioclase and occasional glistening black augites. Under the microscope the porphyritic crystals are seen to be largely plagioclase. The pyroxenes have a pale-brown color in the section and are im bedded in an ophitic groundmass of feldspar laths, magnetite, and chlorite. There is some basaltic brown hornblende not in frequently intergrown with the pyroxene. Diabase. Quite a number of this suite of Alaskan rocks may with pro priety be classed as diabases. These present a variety of structures through which they grade into the augite-porphyrites and ande sites. Indeed, in the absence of all knowledge of the field rela tions, a sharp distinction between these types is impossible. Number 21 of Professor Reid's collection is a dark close grained rock containing many ovoid white spots. The microscope shows it to possess a rather coarse typical ophitic structure with pale gray pyroxene, which is surrounded and supplemented by ex *See Rosenbusch: Mikr. Phys., 2nd ed., vol. ii, 1886, p. 499.
1892 May 15
1892 Feb 19