OK. Dolmens are one a 5 Minute (give or take 30 seconds or so) Cool Down Timer. Once the cool down period is past, anyone approaching the dolmen will trigger it. Geysers are on a similar cool down, but I have not done enough to work out the exact timings.
However, if you hit one Geyser, then go around the island in either a clockwise or counter clockwise rotation, they should come up in order. Although, this really only applies if no one else is doing them, as others may be going in a different pattern and messing things up for you. I have gotten the Geysers to pop as I approach when the zone is otherwise quiet, but normally, its a waiting game.
So, long story short, have patience, you will get to 50 eventually.