46 HAEHL AND ARNOLD — THE MIOCENE DIABASE. [Feb. 6, 



Feldspar. — The feldspars are in two general forms — typical lath- 

 shaped crystals and broader tabular plates with wavy extinction or 

 zonal structure and, usually, an abundance of inclusions. The lath- 

 shaped crystals predominate. As far as it was possible to deter- 

 mine them, both seem to be of the order of labradorite and exhibit 

 the same extinction angles that were characteristic of the plagio- 

 clases of the diabasic type. Individual crystals are seldom over 

 one millimetre in length. Twinning is usually polysynthetic ac- 

 cording to the Albite law, although Carlsbad twins are frequently 

 noted. Flow structure is often beautifully shown by the arrange- 

 ments of the lath-shaped microlites in regular courses between the 

 larger crystals of augite or olivine (see Fig. 23). Not infrequently 

 the ophitic structure of the typical diabase is seen, the feldspars ra- 

 diating, in all cases noted, around the larger crystals of olivine. This 

 is particularly true of the slides cut from the rocks of the narrow 

 dikes. In numerous cases, especially in sections showing flow 

 structure, feldspars are bent and broken and displaced. In many 

 slides the ground mass is badly decomposed and shows practically 

 no optical phenomena, except such as is shown by the feldspar mi- 

 crolites which, in these sections, are so badly etched along the 

 crystallographic outlines that they present rough, saw-like edges 

 (see F, Fig. 23). Feldspars differing from the general type are oc- 

 casionally found in the slides. Slides from one exposure show feld- 

 spars which at first glance might be mistaken for orthoclases, so 

 clear and regular are they and free from banding or twinning. In- 

 clusions are numerous, however, and a closer examination of their 

 optical properties leaves little doubt that they belong to the plagio- 

 clases. There is an unusual amount of analcite in these slides, 

 which suggests very strongly that the feldspars may contain a larger 

 percentage of soda. Again some sections show feldspars, the 

 order of whose interference colors borders closely on nepheline, 

 and one slide shows a number of crystals whose optical properties 

 would tend to class them as melelites. In view of these resem- 

 blances, tests were made upon the thin sections to determine phys- 

 ically whether the optical properties were true indicators. The re- 

 sults, however, left no doubt that the crystals were simply feldspar. 

 Crystals were also found in these slides, portions of which gave the 

 normal optical phenomena of the feldspars common to the rock. 

 Low order interference colors are frequently met with in the more 

 weathered slides,but in nocase could nepheline be positively detected. 



