changing, that rocks are constantly being formed, metamorphosed and disintegrated; that the earth has undergone radical changes, and that the geography of the world in many particulars may be for one generation very different from that for the following one. Much information on the details of these changes is gained from a minute study of rocks and earth materials.
By the aid of the microscope one is able to study the fine points in the relation of different rock materials to one another, and by the aid of the camera exhibit the result to others. As an illustration of this fact Fig. 5 shows the position of a crystal of chloritoid in quartz, and Fig. 6, quartz, crossed by laminæ of mica, taken with crossed Nicols to properly differentiate the materials. These photographs are some of a number made for the Geological Survey to be used in connection with reports. The U. S. Geol. Report for 1899 on the geology of Yellowstone Park shows many photomicrographs, admirably illustrating special geologic features.
Much valuable information is also gained in this way of the structure and properties of metals both in the ore and after smelting and refining. Fig. 7 illustrates the appearance of iron whose tensile strength has been exceeded, and it is easily possible to show also differences in composition or quality of iron or steel by microscopic methods.
The photography of microscopic sections of wood aid very materially in a detailed consideration of forestry. It shows the character of the small tubes or cells of which wood is made up, indicating the definite way in which the cells of the new wood formed each year at the inner surface of the cambium layer are arranged, depending upon the climate where the tree grows. It shows the characteristic differences in cellular structure of different woods, together with the lines separating the growth of successive seasons in the trunk and bark; the medullary rays, which make the silver grain in quartered oak; and a host of other inter-
