The El Camino Real...

...near where it crosses the L.A. Bay highway.
I pulled off between Km. posts 30 & 29, to the north,
on a dirt road. Just beyond where I parked, I found it.
Above photo looking southeast. 29°02.00'/ 113°52.82' (NAD27).

On to El Volcan... Four miles northeast of the El Marmol onyx schoolhouse, the process by which onyx is made can be seen, at El Volcan. Several mineral springs where onyx is created (about an inch every thousand years) are in the arroyo and a quarter mile beyond. The biggest spring is on top of a thirty foot dome, which erupts once a month. This cold water geyser is how El Volcan (The Volcano) got its name. For several minutes water and gas is sprayed 60 feet into the sky! Directions: Take the signed dirt road off Highway 1 for 9.5 miles to El Marmol's onyx schoolhouse. Note sign pointing to the Mina La Olividada road. At 2.0 miles from the schoolhouse, turn right, 4WD recommended from here. At 4.0 miles, come to Arroyo El Volcan. Onyx dome is 0.4 mile to right, up arroyo. Ahead, at 4.3 miles, the La Olividada road passes a white slope with several onyx springs.

Mineral Springs at 29°59.87/ 114°45.87'

If you cross Arroyo El Volcan and continue 0.3 mile on the La Olividada road,
this white slope of tiny onyx springs is passed.

Arroyo El Volcan 29°59.84/ 114°46.13'

At the bottom of the long grade is Arroyo El Volcan (misnamed 'Zamora' in Almanac).
We drove south (in the wash) 0.2 mi., then hiked. Beautiful pools here!

How Deep?

This pool seemed bottomless!