The Fish Lake mountains contain some signs of past volcanic activity, but I'm not entirely sure how they were formed. If anyone can shed light on the geology of these mountains, please post!
I believe that the Fish Lake Hightop is what remains of a huge volcanic plateau from frissure eruptions. Thousand Lake Mountain, Boulder Mountain, and Fish Lake High Tops are all isolated remains of this old volcanic plateau. The mountains are now seperated, and are plateaus, because the basalt caps protect the soft sedementary rocks underneath, and what is left and hasn't eroded away are high plateaus. If not for the basalt caps, the mountains would have eroded away a long time ago.