In my con-world I went lengths to introduce a plausible lighter-than-air element and did rather large amounts of thinking about landmasses in order to promote the development of airship technology over that of land-based technology.
This question is about the geological characteristics of most of the mountain ranges and rock in general in this world. Their mix/composition of rock, sand, dirt, etc.
How would mountains and hills have to be made-up in order to impede tunneling them WHILE still allowing them to be mined out?
By tunneling I mean digging tunnels to be used by trains and land-based-traffic to pass under a mountain rather than having to go up and down again.
By mining out I mean that they contain an abundance of useful minerals and ores that will be mined and make up one of the pillars of industry and society.