The huge bulk of Rainier is built up entirely of closely related lavas, mostly andesite upon a foundation of gray granite. Comparatively little basalt is found although basalt and andesite in their varying forms often merge so closely together that it is difficult to distinguish between them.
Aside from the massive lavas there is much loose material, cinder, bombs, ash and conglomerates, or rather aglomerates, as they were formed by molten lava or volcanic ash flows, which picked-up and cemented together the broken materials which were within their path, but these are mostly only different types of the same basic lava formed by peculiar conditions of ejection or cooling. In places, notably at the point of contact between lava flows or between lava and granite, quartz crystals are found but then again were laid down from the soluble silicas found in the lava.
The base of granite was not uniform in surface and while the average elevation of its surface is between three and four thousand feet above sea level points or peaks are found exposed above five thousand feet.
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