FORMATIONS 219 



accompanying illustration. The sandstone is the country rock, and the 

 diabase forms the rim of the basin within which the alluvium is shown. 



Sandstone. — The sandstone is an important formation in this region, 

 extending to the north and east into the valley of the Wenache river 

 and westward to the crest of the Cascade mountains. It has been given 

 the name of the Swauk sandstone from its occurrence in the Swauk 

 mining district, southwest of Camasland. Its age is Eocene, as deter- 

 mined from abundant fossil plants found in the valley of the Swauk 

 and on Tiptop, the peak in the southwestern corner of the area here 

 considered. 



The Swauk sandstone in the vicinity of Tiptop is a gray arkose, with 

 associated shaly beds which are leaf-bearing. At Camasland and ex- 

 tending eastward the sandstone is white and consists of clean quartzose 

 material. It is massive and not plainly bedded, except where a few 

 thin beds or lenses of quartz pebbles occur. Where the topography is 

 not bold, as within the rim of Camasland, the sandstone is rarely well 

 exposed, but a mantle of quartz sand covers the surface, and only occa- 

 sionally are smooth bosses of the massive sandstone seen. 



Diabase. — The igneous rock of Camasland is a diabase, light gray to 

 brownish gray in color and of varying texture. Its constituent minerals 

 are labradorite, augite, olivine, and magnetite, while its chemical com- 

 position is close to the average for members of the gabbro-diabase-basalt 

 family. For the most part this rock has the ophitic or diabasic texture, 

 but variations from this type occur. Thus on the one side the rock 

 becomes more or less granolitic in texture, and on the other it often 

 approaches the basaltic texture, becoming finer grained and at times 

 even glassy. These textural changes are quite definitely connected 

 with the thickness of the intrusive sheet of diabase. The diabase com- 

 monly weathers into large, rounded blocks, which constitute a notice- 

 able detail of the topogra]3hy. 



Alluvium. — The third formation is the alluvium which forms the sur- 

 face of Camasland itself. This is a rich loam at the surface and contains 

 a considerable amount of organic matter. The few exposures of the 

 subsoil reveal a fine silt with few pebbles. This alluvium covers an 

 area of about one and a half square miles, and its fertility is such as to 

 attract several ranchers, who live in Camasland during the summer 

 months. 



STRUCTURE 



The sandstone in the vicinity of Camasland has participated in the 

 general folding of the region. In the southwestern portion of the area 

 here considered the beds are steeply inclined, and on Tiptop are even 



