500 DESCRIPTIVE GEOLOGY, 



covers a larger area, extending to the southward for two or three miles, 

 where it forms a broad table-topped mass of dark-gray compact rock. It is a 

 remarkably dense 'tough rock, breaking under the hammer with a hackly 

 fracture, unlike those varieties rich in a half-glassy base. It has a fine- 

 grained groundmass, showing under the microscope feldspar and augite. 

 Triclinic forms a quarter of an inch in length, and purely white, are imbed- 

 ded in it; also what would appear as another species of triclinic feldspar 

 with small bladed prisms, a brilliant lustre, and a very evident striation. 

 Olivine, both macroscopical and microscopical, would appear to be wanting. 

 The rock shows a good many cavities and druses filled or coated with car- 

 bonate of lime as a secondary product. 



An analysis of this basalt made by Mr. E. W. Woodward yielded the 

 following : 



Silica - - 54.80 54.79 



Alumina.... 17.58 17.59 



Ferric oxide 0.97 0.94 



Ferrous oxide 8.84 8.85 



Manganous oxide trace trace 



Lime 8.22 8.13 



Magnesia 4.47 4.54 



Soda 3.14 2.97 



Potassa.. 1.16 1.16 



Water and carbonic acid 0.94 0.98 



♦' 



100.12 99.95 

 Specific gravity 2.5, 2.6. 



It will be seen that the analysis runs very high in silica ; indeed, no 

 basalt analysed from our collection gave so large a percentage. It is cor- 

 respondingly low in its specific gravity. 



Along the east base of Pilot Peak, unrepresented on the map, occurs 

 a small outflow of gray porous rhyolite. 



In the Quaternary deposits that surround the Ombe Eange, there is 

 but little that is distinctive. On the east side of Pilot Peak, beyond the 

 long slopes of Upper Quaternary accumulations, occurs a broad level plain, 



