This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Trap rock, or diabase, is a crystalline, granular rock, composed essentially of feldspar and augite; but nearly all contains magnetite and frequently olivine. They are basic in composition and in structure; they are, as a rule, massive. The texture, as a general thing, is fine, compact, and homogeneous. The colors are somber, varying from greenish, through dark gray, to nearly black. Owing to its lack of rift, its hardness, and its compact texture, trap rock is generally very hard to work. It has been used to some extent for building and monumental work, but is more generally used for paving purposes. Within the last few years, on account of its great strength and fire-resisting qualities, it has been extensively used in concrete work. The crushing strength of trap rock or diabase is usually between 20,000 and 26,000 pounds per square inch.