16. Conglomerates. Conglomerates differ from sandstone only in structure, being coarser and of a more uneven texture. The grains are usually an inch or more in diameter.
17. Granite. The essential components of the true granites are quartz and potash feldspar. Granites are rendered complex, although the essential minerals are but two in number, by the presence of numerous accessories which essentially modify the appearance of the rocks; and these properties render them important as building stone. The prevailing color is some shade of gray, though greenish, yellowish, pink, and deep red arc not uncommon. These various hues are due to the color of the prevailing feldspar and the amount and kind of the accessory minerals. The hardness of granite is due largely to the condition of the feldspathic constituent, which is valu able. Granites of the same constituents differ in hardness.
Granites do not effervesce with acids, but emit sparks when struck with steel. They possess the properties of strength, hardness, and durability, although they vary in these properties as well as in their structure. They furnish an extensive variety of the best stone for the various purposes of the engineer and architect. The crushing strength of granite is variable, but usually is between 15,000 and 20,000 pounds per square inch.
lS. Trap Rock. Trap rock, or diabase, is a crystalline, granu lar rock, composed essentially of feldspar and augite; but nearly all contains magnetite and frequently olivine. They are basic in com position 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 hick of rift, its hardness, and its compact tex ture, 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. NN ithin 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 dia
base is usually between 20,000 and 26,000 pounds per square inch.
19. Seasoning of Stone. Stone, to weather well, should be laid with its bedding (lamination) horizontal, as it was first laid down by nature in the quarry. The stone, moreover, will offer greater resistance to pressure if laid in this manner, and, it is said, will stand a greater amount of heat without disintegrating. This is important in cities where any building is liable to have its walls highly heated by neighboring burning structures.
Some stones that are liable to be destroyed by the effects of frost on first being taken from the quarries, are no longer so after being exposed for some time to the air, having lost their quarry water through evaporation. This difference is very manifest between stones quarried in summer and those quarried in winter. It has frequently happened that stones of good quality have been entirely ruined by hard freezing immediately after being taken from the quarry; while, if they are quarried during the warm season of the year and have an opportunity to lose their quarry water by evapora tion prior to cold weather, they withstand freezing very well. This particularly applies to some marbles and limestones. This change is accounted for by the claim put forward, that the quarry water of the stones carries in solution carbonate of lime and silica, which is de posited in the cavities of the rock as evaporation proceeds. Thus additional cementing material is added, rendering the rock more compact. This also will account for the hardening of some stones after being quarried a short time. When first quarried they are soft, easily sawed and worked into any desirable shape; but after the evaporation of their quarry water, they become hard and very durable.
Table I gives the physical properties of many of the most im-. portant varieties and grades of building stone found in the United States.