Make sure the environment settings for the snow is on 'high altitudes' at about 75-85, and on 'steep slopes' about the same. Then when you use the mixing slider between material 1 and material 2, it will go up or down.

The other way and one which gives you different controls is by using layers first. If you take the rock and add a new layer of snow, then you'll see the environment tab all the way to the right. In there are some different controls...I like the two way altitude slider and the fuzzy sliders. 

What I like to do is mix my materials first, save them and then use them in layers.....

Be careful though....I've been lost in the material editor for days on end with little food or water and no contact with the outside world....barely made it back and scarcely scratched the surface of what can be done in there.....right now I'm exploring the variety gained by using the filters.

That said, if we don't hear back from you...we'll know where you are lol....

M