I've been told that this interesting very black seam along CA-178 is likely to be volcanic in origin.

Following is an edited (with his permission) of an email from SLGWV about this seam:

It looks like a cooling unit (vitrophyre is the $64 word) enclosed within an ash-flow tuff, the pink rock above and also below. An ash-flow tuff is a volcanic rock that was erupted as hot particles entrained in extremely hot gas, such that the whole business flows like a fluid (i.e., a density flow). This appears to be a welded tuff (aka ignimbrite), where the particles are still hot enough to weld together when they settle out. There are lots of enormous ignimbrite units in the area, of late Tertiary age (say within the last 20 million years), so it fits with the regional geology. Thus, that black unit is basically an obsidian, a dark volcanic glass. This would be easy to distinguish from coal with a rock hammer! ;)

The gray rocks in the foreground, this side of the highway, are Paleozoic limestones--much older rocks.