In this image is visible a rock formed by clasts of the Calcari a Campionella Fm. (late Cretaceous), of the porphiry of San Martino (8 Ma) and a matrix formed by the Palombini Shales (late Cretaceous).
This was caused by the heat of the magma of the Porphiry of San Martino which caused a violent boiling of the water in the shales. The resulting explosion caught clasts from the near limestones and from the solified magma and the shale liquified flowing around the clasts. Then the breccia re-lithified.
PhotoID599