Mark Bibbo spotted these plants on a rocky ledge while we were botanizing on San Bruno Mnt. The conspicuous scarious stipules hinted at Caryophyllaceae, and from the species list in the ''Flora of San Bruno Mountain'', Paronychia franciscana seemed the likely candidate (and that's indeed what it keyed to in Jeps). From the epithet, I initially thought it would be a California native. But I was surprised to find otherwise. Here's an interesting quote from the Flora of NA treatment:

''Although Paronychia franciscana was described from California, where it has been known from the San Francisco area since 1887, the species is native in Chile.''