The Forgotten Planet by Murray Leinster

The sun that shone upon the forgotten planet was actually very near. It shone on the top of the cloud-bank, and the clouds glowed with dazzling whiteness. It shone on the mountain-peaks where they penetrated the mist, and the peaks were warmed, and there was no snow anywhere despite the height. There were winds, here where the sun yielded sensible heat. The sky was very blue. At the edge of the plateau -- from which the cloud-banks were down instead of up -- the mountainsides seemed to descend into a sea of milk. Great undulations in the mist had the semblance of waves, which moved with great deliberation toward the shores. They seemed sometimes to break in slow-motion against the mountain-walls where they were cliff-like, and sometimes they seemed to flow up gentler inclinations like water flowing up a beach. But all of this was very deliberate indeed, because the cloud-waves were sometimes twenty miles from crest to crest.

The look of things was different on the highlands. This part of the unnamed world, no less than the lowlands, had been seeded with life on two separate occasions. Once the seedings was with bacteria and moulds and lichens to break up the rocks and make soil of them, and once with seeds and insect-eggs and such living things as might sustain themselves immediately they were hatched. But here on the highlands the different climatic conditions had allowed other seedlings and creatures to survive together.