72 11. F. Reid — Stadies of Muir Glacier. 



ill 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 

 Avhose porphyritic crystals are neither so pronounced nor so 

 abundant. Its groundmass is a network of panidiomorphic 

 plagioclase laths connected by a mesostasis which was prolxibly 

 once a glass, but which now is a brown, extremely fine-grained, 

 but brightly polarizing mass carrying chlorite and secondary 

 amphibole. Magnetite is also al)undant. 



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-broAvn 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 intergroAvn 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 tliese 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 Eosenbusch : Mikr. Phj^s., 2nd ed., vol. ii, 1886, p. 499. 



