Can you tell by looking at the soil whether the underlying bedrock is granite or basalt?
Considering the fundamental differences between the two types of rock, it should be possible to tell the difference, especially if the soil is relatively barren of humus and other "contaminants." Granite is felsic, and should produce relatively light colored, quartz-rich soils, while basalt, being mafic, can be expected to produce darker (possibly redder) iron-rich soil horizons. However, there are so many variables involved that it would be dangerous to base a bedrock determination solely on soil type - it's always best to get a look at some outcrop.
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