Its great. I don't know the scale but as a set of islands in a world then the mountains could be very large. If it was similar to Hawaii or a small island set then they would be about right. The shape of them is very nice.
I am not sure about the white lines. I know many people put them on their map and I think there is some method to their position and direction and your lines might be correct, but it looks a little confused in the middle where they cross over each other.
Good use of color and texture too.