LITHOLOGY. 219 



tening in its lustre, and rather soapy in its feel. I analyzed the rock, 

 and the following was the result: 



Silica, 60.49 



Alumina, ........... 19-35 



Iron sesquioxide, .......... .48 



Iron protoxide, .......... 5.98 



Lime, 1.08 



Magnesia, 2.89 



Soda, 2.55 



Potash, ............ 3.44 



Water, 3.66 



99.92 



This analysis indicates that the rock has the composition of an ordi- 

 nary clay, minus the larger part of its water. It is plain that, acccording 

 to the efficiency of the forces that act in recrystallizing such sediments, 

 material of this composition could be converted into a slate, a granite, a 

 porphyry, or one of several other rocks. That such rocks as these 

 schists are particularly characteristic of the valley, is to be referred to 

 the feeble action of the same metamorphic agencies that in the interior 

 regions have created the coarsely crystalline type, which here is largely 

 replaced by the fine-grained and the half fragmental type. 



When a thin section of this schist is examined with the microscope, it 

 appears to be made of an extremely fine aggregate, but with a high mag- 

 nifying power it is seen to be composed of w^hite transparent quartz and 

 a fine foliated substance, most of which is apparently mica in a very fine 

 state of division. These parts are brought into sharpest relief in polar- 

 ized light, for the micaceous constituent assumes the brightest colors. 

 As the section is revolved between crossed Nicols, the foliae become dark 

 when in the plane of either Nicol, as does mica, and as the folias are 

 mostly in one plane they nearly all become dark at once. The examina- 

 tion of the section shows besides, that some green chlorite, some trifling 

 bits of calcite, and minute grains of iron oxide are scattered about, and 

 some bits of the granular substance have the white semi-transparent 

 appearance of feldspar. A section of this rock, as it appears when mag- 

 nified 600 diameters, is represented in Fig. 2 on PI. 12. It is introduced 

 to show that these rocks, though so fine in texture, are composed of 



