1904.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 41 



Structure with smaller individuals, yet of nearly the same chemical 

 composition. While no separation of the two has been attempted 

 in mapping in the field, they will be treated under separate heads in 

 dealing with their microscopic character. In describing their field 

 relations the first has been called the diabasic, the latter the basaltic 

 facies. It is not intended that these terms shall be used to desig- 

 nate two distinct series of rocks, but rather in the sense of a con- 

 venient classification of two facies of the same magma. 



Fig. 20. Thin section of the diana-A lacits i^Ni-ecimen 24), showing the typical 

 diabase structure. (A), augite; (F), feldspar; (M), magnetite. X 20, 

 Photograph by Ralph Arnold. 



The Diabasic Facies. 



Considering the general tendency of the eruptive to disintegrate, 

 the diabasic type is usually remarkably fresh and clear in thin sec- 

 tions. The slides show the following principal constituents, given 

 in the order of their crystallization : magnetite, ilmenite, apatite, 

 olivine, feldspar, augite and analcite. The last is never present as 

 an original constituent, so far as could be determined, but is cer- 

 tainly in many cases, and probably in all cases, a secondary pro- 

 duct. Of the secondary minerals, serpentine, chlorite, iron ores, 

 calcite and natrolite have been noted. 



Plagioclase. — The feldspar is generally present in the diabasic 

 facies in rather stout, lath-shaped forms with an average length of 

 two millimeters, twinned according to the Albite and Carlsbad laws 



