Petrography lower if wholly or partly vitreous. The specific gravity of the more common rocks range from about 2.5 to 3.2. Archaeologists use petrography to identify mineral components in pottery. This information ties the artifacts to geological areas where the raw materials for the pottery were obtained. In addition to clay, potters often used rock fragments, usually called "temper" or "aplastics", to modify the clay's properties. The geological information obtained from the pottery components provides insight into how potters selected and used local and non-local resources. Archaeologists are able to determine whether pottery found in a particular location was locally produced
A Study of History 23 civilizations identified by Toynbee in vol. Vii. This table does not include what Toynbee terms primitive societies, arrested civilizations, or abortive civilizations. Civilizations are shown in boldface. Toynbee's "Universal Churches" are written in "italic" and are chronologically located between second- and third- generation civilizations, as is described in volume VII. The social scientist Ashley Montagu assembled 29 other historians' articles to form a symposium on Toynbee's "A Study of History", published as The book includes three of Toynbee's own essays: "What I am Trying to Do" (originally published in "International Affairs" vol. 31, 1955); "What the Book is For: