726 DESCRIPTIVE GEOLOGY. 



ments of feldspar. An analysis of this rock, made by Mr. R. W. Woodward, 

 yielded: 



Silica •- 69.52 69.84 ' 



Alumina 15.82 15.60 



Ferric oxide 1.74 1.80 . 



Ferrous oxide 1.07 1.07 



Manganese — trace trace 



Magnesia 3.13 3.14 



Soda 0.48 



Potassa 5.39 



Ignition 3.34 3.36 



100.49 

 Specific gravity, 2.7. 



In its chemical composition, the essential features of this rock are quite 

 like the porphyroid, the analysis of which has already been given. It 

 carries no lime; the percentage in iron is somewhat low, and, like the other 

 rocks from the region, shows the same preponderance of potassa over soda. 



This last formation is immediately overlaid by black arenaceous slates 

 containing more or less calcareous material, which are but two or three hun- 

 dred feet thick. They dip about 40° to the west, and trend a few degrees 

 east of north and west of south. These black slates are again overlaid by 

 very thick limestones, which cannot be less than 1,800 or 2,000 feet thick. 

 This is much fissured and stained by oxide of iron and traversed by narrow 

 seams of calcite and quartz. It is highly metamorphosed, and in places is 

 completely converted into white, coarsely crystalline marble. It has been 

 less searched for fossils than the other belts, but those found have been 

 recognised as belonging to the genus Ammonites, and probably to species 

 already described. This series is again followed by another quartzite and 

 interstratified siliceous slates, which form a steep bluff-like face on the east 

 side of Star Peak extending nearly to the summit; but the actual top is 

 formed of black and gray limestone. This is probably the same zone that 

 occupies the foot-hills just north of Echo Canon on the west side, and the 



