622 ME. H. H. AENOLD-BEMEOSE ON THE MICROSCOPICAL [Nov. 1 894, 



angles one to another, and the direction of extinction bisects the 

 angles between them. These are nearly perpendicular to the 

 C axis. Others have fine parallel cracks which often run in the 

 direction of the length of the plates. The}' appear in the different 

 portions into which a plate is divided by the felspars. If they re- 

 present the usual prismatic cleavages, we have sections out of the 

 prism zone. The augite often extinguishes at large angles with 

 these cracks. After measuring a number of angles of extinction, I 

 find that the largest which the axis of least elasticity of the section 

 makes with the cleavage-cracks are 40°, 42°, 45°, and 52°. The 

 angle c/\y is therefore 52° or more. Amongst the opbitic plates are 

 many twins. In some cases, both individuals possess cleavage-cracks 

 parallel to the trace of the plane of composition, and extinguish 

 symmetrically with regard to it. These sections are out of the 

 prism zone, and the plane of composition is (010). I have measured 

 the angles which the direction of extinction of each individual 

 makes with the twinning-line, and the greatest I have found is 45°. 

 In these plates the angle c/\y is 45° or more. 



Some small plates of augite enclose one or two felspars, or have 

 the end of a felspar sticking into them. They sometimes occur 

 with the ordinary granular augite, and are distinct from the true 

 ophitic structure. 



The phenocrysts are of the usual form, and the sections are often 

 bounded by six or eight sides. The largest measured is 1*65 x 1'20 

 mm. Some of the biggest crystals are corroded, and others contain 

 portions of the groundmass. The hour-glass and the zonal struc- 

 tures are very frequent. The outer portion of an individual of a 

 twin crystal often extinguishes differently from the inner portion. 

 As in the case of olivine, there are many groups the individuals of 

 which can only be distinguished in polarized light. The smaller 

 phenocrysts are similar to the larger ones, and their boundaries 

 quite as well defined. 



The lath-shaped sections have a well-defined outline, and some- 

 times cleavage-cracks parallel to their length. They often extin- 

 guish when the cross-wire is inclined at an angle of about 45° with 

 their length. They are distinguished from untwinned felspars by 

 their polarization-colours. I have measured a number of angles 

 made by the least axis of elasticity with the long axis of the prism, 

 and the following are the greatest : 41°, 42°, 43°, 45°, 45°, 45°, and 

 46°. Some of them are twinned. The grains vary in size, and are 

 irregular in shape. The phenocrysts, lath-shaped sections, and 

 grains, or the two former only, occur sometimes together in the 

 same thin section, so that we have two generations of augite. 

 Lath-shaped sections and grains occur together, but most often the 

 grains occur alone. 



6. Felspar. 



The felspar, which is triclinic, occurs in two generations. Of the 

 fifty-nine largest crystals measured, one is over 3 mm. in length, 

 ten are over 2 mm., seventeen are over 1 mm., twenty are over 5, 



