PLATE XIII. 



(Sp. No. 23644. Natural size.) 



This illustration is a faithful representation of the appearance of the polished surface of a 

 pyroclastic from the Hemlock formation. It is somewhat doubtful whether or not the fragments 

 composing the rock have been deposited through the mediation of water or air alone. The larger 

 fragments are rather dense. Vesicular fragments are more common among the smaller particles. 

 Pyroclastics similar in appearance to this are of very common occurrence in the Crystal Falls district, 

 and huge cliffs of it are readily accessible from the railroad. 

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