Take a look at two nearby spots where you can collect large scale crystaline minerals. One is Shell Canyon near Ocotillo Wells. The other is Wiley Wells. Here's what they look like in Google Earth.
They look pretty similar to me. Exposed rocks jutting out over a sedimentary layer. Here are their positions.
Wiley Wells 33°27'5.87"N, 114°53'36.11"W
Shell Canyon 32°46'31.94"N, 116° 0'31.22"W
What's going on here? Why is it that the crystals are so big here and not in other places? Is this a batholith that has been exposed by erosion? I haven't been to Wiley Wells in person, but in Shell Canyon you can easily find quartz pieces in sizes almost too big to lift.
Since different minerals solidify at different temperatures, my first thought is that here the magma cooled slowly, allowing the quartz to crystalize in huge lumps while the remaining rock remained liquid. Does that make sense?
Let's see what our Keeper of the Catacombs has to say...