155. Marble is simply a crystallized limestone, capable of taking a good polish.

The scarcity and consequent expense of good marbles have in the past prevented them from being used in constructional work, except occasionally for columns. Most of the marbles obtained from the older quarries also stain so easily that they are considered undesirable for exterior work.

Since the rapid development of the Georgia and Tennessee marble quarries, however, stone from these quarries has been much used for exterior finish, and even for the entire facing of the walls. These marbles will probably be more extensively used for exterior work in the future, as they are exceedingly strong and durable and do not stain readily.

Nearly all varieties of marble work comparatively easy, and the fine-grained varieties are especially adapted for fine carving.

They generally resist frost and moisture well, and are admirably suited for interior decoration, sanitary purposes, etc., and in clear, dry climates make a splendid material for exterior construction.

The strength of marble varies from 5,000 to 20,000 pounds per square inch, and only when used for columns need its strength be considered.

[For the composition and strength of various marbles see tables in appendix.]