1904.] 



HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



49 



and 0\ Fig. 23). It shows high polarization colors and is strongly 

 pleochroic in the green shades, the greatest absorption being paral- 

 lel to the fibres. It agrees strongly with the mineral described by 

 Iddings^ from Nevada and afterwards named iddingsite by Lawson. 

 A small amount of chlorite was noted. Calcite is quite abundant 

 in the more weathered portions of the rock. It is usually found 

 filling seams, joints and amygdular cavities. 



Fig. 25. Natrolite (N) and analcite (A) between crossed nicols. x 3^. 



Analcite. — The most striking product of decomposition is anal- 

 cite. Its occurrence in the field has been described. In certain 

 areas it is quite plentiful. In thin section it is often found associ- 

 ated with natrolite, fibrous aggregates of which it frequently in- 

 cludes (see Fig. 25). It is isotropic, occasionally showing optical 

 anomalies. It has been observed to occur in five general ways : i . 

 In irregular patches in the centre of crystals of plagioclase. 2. In 

 a form suggesting a decomposition product of the plagioclases, ad- 

 vancing in irregular lines from the crystal edges inward. 3. Com- 

 pletely filling what seems to have been the rectangular outline of a 

 plagioclase crystal. 4. In irregular patches filling the angular 



1 " Geology of the Eureka District, Nevada," by Arnold Hague, Mon. XX, 

 U. S. G. S., Appendix B, pp. 388-390, Washington, 1892. 



PROC. AMER, PHILOS. SOC. XLIII. 175. D. PRINTED APRIL 2, 1904. 



