Tuff stuff.

A small excavation in Vinoland has revealed a friendly, neighbourhood igneous rock (everybody should have one).  Tuff, or ash-fall tuff, is extremely variable in its chemical and mineralogical composition.  Vinoland's tuff happens to be extremely light in weight and texture, but there exists some historical references (The Structural and Industrial Materials of California, Bulletin No. 38, State Mining Bureau, 1906), to a brecciated tuff, mined just a mile or so from here, that "...for many years has been used for macadamizing the streets of Napa."  Now, that's tough stuff.
Tuff is a fine grained rock composed of volcanic ash which fell upon this part of the Napa Valley millions of years ago, (vulcanism reached the valley about 7 million years ago), which later hardened into beds of tuff.  Vinoland's tuff is really light in colour (it resembles diatomaceous earth in appearance, though it is denser), includes no clasts (volcanic detritus), rubs off on your fingers like chalk and is usually high in silica. 
Tuff is only found in a small area in the south end of Vinoland, (some Syrah is planted in it, tough stuff for a grapevine to deal with).  I know that at least one of our neighbours over the hill behind Vinoland has a large amount of it, but I don't know how extensive it is in the neighbourhood as a whole.  Some folks claim that the volcanic rock in the Napa Valley is responsible for certain characteristics in the grapes and resulting wine.  However, due to the huge diversity in the types of volcanic rock deposited in the length and breadth of the valley, I am of the opinion that there is little significance you can attribute to just one specific lithology.