Thanks so much for this info. Amboy is about 2.5 hours east from where I'm located, and I've often wondered about it, but haven't yet gone out there. Looking at the Google Map satellite views of the point where you leave Cadiz Rd and turn onto the dirt road (which ends up paralleling the RR), there appears to be another dirt road running NNW, which terminates ~3/5ths of a mile later, at what Google Maps is calling "Camp Ground for Marble Mountain T634".
Obviously, this is a completely primitive campsite. Any idea whether this is a privately-owned or otherwise somehow "restricted" campground? If not... it certainly seems perfect for an observing site.
This is BLM land, not private. Pretty much all but a couple of chunks of the mountain fall under it (some mining companies possess some parts of the hillsides on the back and eastern end, the Vaughn Mine is long-gone).
I think the T634 label is, amusingly, for a Boy Scout Troop- the first time I visited the area, big group camped out (to dig up fossils) and coincidentally parked right where X marked that spot. The dirt road leading to it visible on Google Earth is the exact one you take and if you scroll up, you'll see another branch off at 90 degree mark that curves up and into the mountain, right to the trilobites (NOT a good road for small vehicles- I managed to get my Corolla up the hillside to the amazement of some geologists and never plan to repeat that!)
It's precisely 2.5 hours drive-time from Rancho Cucamonga, so it has made for some great adventures for my son and I. I move back East in February so this Saturday marks our final excursion out there, sadly. It has been a wonderful spot.