In the long term, a number of materials presently in common use will probably become exhausted, notably some metals and some varieties of stone, but other materials will adequately replace them. The raw materials for concrete, glass bricks and aluminum are available in very large quantities, although the cost may increase if less economic raw materials have to be used. Prior to 1950, steel frames were used for most high-rise buildings because they usually cost less than reinforced concrete frames. Since then, concrete has become more competitive, but this has not been a steady progress. Although the majority of tall buildings in Europe and Australia are now built with concrete frames. this is not the case in North America. The choice has thus varied with time and geographic location。 Cost remains a principal criterion of choice, particularly if a material will not be visible in the completed building. There is, for example, little difference between the performance of structural steel frames and reinforced concrete frames if they are properly designed. The cheaper of the two is usually selected, making due allowance for the effect of earlier or later completion time on cost and profitability.