EDIT: To step by step it:
1. You have your line art on a transparent layer if you've drawn it via tablet/stylus, or you can separate it from a scan and arrange it thus.
2. Select > Load Selection OR Right-click the layer preview > Select Pixels.
3. Use the Lasso-Select (add to) tool to select the bottoms of your mountains from either side of the base of the line art outline. (Click and drag from one end to the other on a downward curve to include the base of the mountain). The selection will automatically close in a straight line on top.
4. With that selection open, create a new layer and give it a new mask with that selection. Fill the layer (not the mask) with a color so you can see the extent of the mask.
5. With the magic wand tool ('contiguous' and 'sample all layers' checked) select the background (everything outside the mountains).
6. Create a new layer below the line art and give it a new mask with that selection. Fill the layer (not the mask) with a color so you can see the extent of the mask. Invert it. Be sure to disappear the top layer so you can see everything.
7. Note, any mountains with a break in the top outline of the line art will not be fully masked; you'll have to paint those areas by hand (on the mask, with white color and a hard brush). Do that now.
8. Adjust the mask edge (in the masks panel). Contract the mask until it conforms to (disappears from the outside of) the line art outline.
9. Use a soft, low opacity brush on the mask to smooth out the bottom edge of the mountains.
10. Done. It takes longer to explain than accomplish.