Dar al-Pelor

[ravells]

I'm not a gimp user, but try using a layer mask and either soft black brush with a low opacity along the edges or a cloud filter on top of the layer mask, using a white brush to fill in the central bits you want to remain opaque....shout if you'd like more detail on this.

The first image is a hard edged texture fill. I've made a selection and filled it with a mountain/rock texture against a background terrain texture.

In the second image I've used a layer mask and a soft edged black brush to blur the edges.

In the third image I've used a 'jiggle filter' (free download, check the editable map tutorial in my sig for the address)

In the fourth image I've made a layer mask isolating the mountains and then blurred it using a gaussian blur.