art, 12. SIDERITE AND ASSOCIATED MINERALS—SHANNON. a 
greater part of Washington, all of eastern Oregon, much of northern 
California, and the great expanse of the Snake River Plain in Idaho. 
The approximate extent of the lava fields is shown in the accom- 
panying map (fig. 1). 
It is generally believed that the lava was poured out of open fis- 
sures and, being exceedingly fluid, spread widely in thin level sheets. 
This idea is based upon several lines of evidence: First, the volume 
and extent of the lava are so great that it is difficult to conceive 
their eruption from ordinary craters. Second, the rarity of the 
fragmental materials known as tuffs, breccias, etc., indicates a well- 
ing up through fissures without the explosive action characteristic of 
crater eruptions. ‘Third, old fissures through which the floods of lava 
came forth have been discovered, fissures now filled with cooled and 
hardened basalt in observable connection with the overlying sheets 
of lava. 
The basalt area is not a single flow but rather a complex of nu- 
merous relatively thin flows, the whole reaching in places a thickness 
of thousands of feet. About 20 separate flows are exposed in bluffs 
of the Columbia River. Each one represents a distinct outpouring 
of lava, the eruptions being separated by long periods of time, some 
of which must have endured for centuries, for they are marked by 
beds of soil in which trees grew to considerable size before being 
charred and buried by later flows. The eruptions of lava, especially 
in the later stages of activity, were separated also by periods in which 
other materials accumulated, consisting of volcanic ash and beds of 
sand, clay, and gravel laid down in lakes and rivers. 
In addition to the massive flows that constitute the greater part of 
the material there are beds of fragmental volcanic material, such as 
bombs and smaller angular fragments of lava. Such materials have 
either been thrown up to great heights and rained down upon the 
surrounding country or, mixed with hot water, have flowed over the 
surface as mud. The fragmental materials are not as widely dis- 
tributed as the massive flows. 
The quantity of material poured out during this period was enor- 
mous. The greatest thickness of the lava and associated deposits is 
not less than 4,000 feet (1.25 kilometers), but if it averages only 
500 feet (0.15 kilometer) over its entire area, seemingly a very mod- 
erate estimate, it would make a mass of 24,000 cubic miles (100,000 
cubic kilometers) or a cube nearly 30 miles (48 kilometers) in height. 
Even this great volume may be far less than was actually poured out. 
GEOLOGY OF THE CITY OF SPOKANE. 
The geology of the city of Spokane has not been mapped in detail, 
although many geologists have worked in the surrounding territory 
and have examined the rocks in the city casually. The basalt is 
