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The most celebrated mines of Mexico and Peru are found in the primitive and transition schists, in the trap-porphyries, the grauwakke, and the alpine limestones.
The secondary formations gently disappear, and are succeeded by the transition rocks, which indicate the proximity of primitive strata. We found considerable mountains composed of that ancient grey stone which the mineralogists of the school of Freyberg name grauwakke, and grauwakkenschiefer.
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