Bulgarian Stone Collections

building stones are variously classified according to geological position, physical structure, and chemical composition. the geological position of rocks has but little connection with their properties as building materials. as a general rule, the more ancient rocks ate the stronger and the more durable; but to this there are many exceptions. according to the usual geological classification, rocks are divided into igneous, metamorphic, and sedimentary. greenstone, basalt, and lava are examples of igneous rocks; granite, marble, and slate, of meta morphic; and sandstone, limestone, and clay, of sedimentary. al though clay can hardly be classed with building stones, it is not entirely out of place in this connection, since it is employed in making bricks and cement, which are important elements of masonry.