42 HAEHL AND AKNOLD — THE MIOCENE LIABA.SE. [Feb. 



Fig. 21. Twinned crystal of labradorite, showing cleavage, X 60. 



(see Fig. 21). Of the two, the Albite twinning predominates and 

 is usually polysynthetic, and occasionally combined with the Per- 

 icline. Crystals cut parallel to the composition plane and showing 

 tabular forms are not infrequent, and in many cases show zonal 

 structure and wavy extinction, indicating a centre more basic than 

 the periphery. Extinction angles were carefully taken and indicate 

 plagioclases of about the order of labradorite with a formula of 

 mixture about AbjAn^. 



Decomposition ot the feldspars has gone on to a great extent in 

 portions of the mass. Comparatively fresh sections are obtainable, 

 however, in places. Kaolinization is very common in all sections. 

 The most characteristic alteration, however, seems to be that which 

 results in the formation of analcite within the feldspar. Nor does 

 it seem that any one law of decomposition applies to all the cases 

 seen. In one instance a m.ere patch of an isotropic, clear glassy min- 

 eral is found in the centre of a plagioclase. In another the crystal 

 form of the feldspar appears to be filled with the product, except, 

 perhaps, a small fresh patch of the original mineral left in the cen- 

 tre, in just such a position as the analcite held in the first case cited. 

 Occasionally the whole crystal is replaced by the analcite. In the 

 slides examined there seems to be much evidence that the analcite 

 is an alteration product (in these instances) of the feldspars them- 

 selves. The problem of the percentage of soda required for the 



