Combining two different material in to one in node editor I liked to combine two different materials in to one material in nodes, so that 
I could create mountain like structures.

I try to use the noise as the base of the mountain structure, and add a different snow like material to the top of the hills where the normal height is high in the noise texture.

How do I combine two different material on top of each other, so that the height decides what material is used?
For your mountain example, you can use position attribute in geometry input node as a factor for high in Mix Shader or MixRGB (depend on using shader or using  texture).

In the other way, you can use weight paint for highlighting the area you want, and then use Layer Weight node as a factor.