SanLuisObispoGlass

A specimen of brown glass of secondary volcanic origin. Approximately 1-1/2 inches across.

I believe this layer of glass approximately 1/4 inch thick was formed by the melting of limestone in contact with the magmatic material forming the rhyolite plugs which are prominent in San Luis Obispo.
The most notable of these rhyolite plugs is the well known Morro Rock formation in Morro Bay.
The glass is found in small fragments in great profusion in Corralina Cove, several miles south of Morro Rock in the Montana De Oro State Park. Fragments of glass firmly cemented between limestone layers like the one in these photographs are more unusual but not difficult to find.
There are some specimens where one of the limestone layers has been worn away and on the surface of the glass formerly cemented to the limestone can be seen spherulites of recrystallized material which seem to have formed via redeposition from supersaturated solution.

The bounding limestone is strongly cemented to the glass layer, and is preferentially worn away due to it's softness. One of the limestone layers is mostly worn away in the specimen pictured here. The glass itself is quite soft, and fragments found in the surf are fairly irregular in shape, present as tabular pieces 1/4 to 1-1/2 inches in the longest dimension.

Examination of numerous fragments leads me to believe that the thickness of the layer is very regular and that the 1/4 inch thickness of the glass layer in this specimen is typical. The color of the glass and the bounding limestone layers is well represented in the photographs. Note the glassy fracture surfaces, crystalline characteristics are absent.

I was not able to detect the glass layer in the exposed limestone cliffs of the cove, but it is undoubtedly there, since there is a much larger proportion of the brown glass fragments in Corralina Cove than in the nearby adjacent cove, the pebbles of which otherwise resemble those of Corralina.