Sonoma County
This rock differs from diabase only in point of geological age, being a product of post-Tertiary eruptions. ... Sandstone and Tafoni ... . It is a fine-grained, stratified, porphyritic, dark gray trachyte. Near these block quarries the same kind of rock has been quarried to some extent for curbing stones, and slabs, or blocks of it eight to ten ...Web
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