220 University of California Publications. [Geology 



of the third is. If this explanation is the correct one, then the 

 thermal conductivity of this rock is very definitely merely the 

 integrated effect of the thermal conductivities of its constitu- 

 ent minerals, and the structure of itself is of no consequence. 



A brief description of the other constituent minerals of the 

 rock is essential in order completely to individualize it. 



Quartz (x) occurs in large lenses in large fragments. A 

 cross section of these lenses may be as much as .2 by 3.0 cm. 

 The individual fragments show no uniformity of orientation, 

 and some of them show a greatest dimension of .3 cm. A wax 

 figure that covered a part of such a large fragment would natur- 

 ally be unsymmetrical. The non-uniformity of orientation of 

 the fragments of these lenses is shown by the diagram fig. 8, 

 the result of a few minutes of work with the quartz wedge. The 

 interference color shows how nearly the axis of elasticity is per- 

 pendicular to the section, and the line shows the projection of 

 this axis on the plane of the section. 



Fig. 8. 



The quartz (y) and the feldspars give the rock its uniform 

 allotriomorphic granular character. There is little variation in 

 size of the grains. Albite twins are scarce but not rare. The 

 extinction angles of the twins, measured from the traces of the 

 planes of composition, are low. This latter fact and the uniform 

 gray blue tints of the feldspars indicate the absence of any pla- 

 gioclase more basic than oligoclase or andesine. If we remember 

 that in non-igneous rocks the albite twins of the feldspars need 

 not be well developed, we find it impossible to separate the feld- 

 spars into orthoclases and plagioclases. Quite a few carlsbad 

 twins are present, however, and these tell us something. 



The quartz makes up about % of this aggregate of feldspar 

 and quartz. It is easily recognized by its yellow interference 



