Photography has now become so invaluable an accessory to every branch of science, that it is not surprising to find it has proved indispensable alike to the geologist and the palæontologist. One has but to turn back over the four decades of this Magazine and compare the number and quality of the illustrations, in the earlier with the later years, in order to realize the great advances that have been made by the application of photography both to process-plates and to illustrations in the text (especially in the last quarter of a century), which has so enriched and enhanced the value and interest of this and other scientific journals. That good half-tone process-blocks can be made from photographs of any geological subject taken in the field, is now well established; nevertheless, two points are absolutely essential to their success: (1) the photograph must be particularly clear and well-defined in its details; (2) the block, however good, may easily be rendered worthless by bad or careless printing; moreover, it must be reproduced upon paper with a finely finished surface, although it need not necessarily be the highly glazed kind (heavily loaded with kaolin) now so much in vogue for the illustrated Monthlies.