THE fifty-sixth supplement to Petermann's Mittheil-ungen has just been published, and consists of a masterly treatise on-Deltas by Dr. G. A. Credner, of Halle. The author shows the importance of deltas in reference, both to geography and geology, and discusses carefully the real import of the term. He them, in the first part of his work, treats of the Formation, Structure, Growth, and Distribu tion of Deltas under the heads of (1) Limit and Form of the Delta; (2) Formation and Condition of the Delta Surface; (3) Size of the Delta; (4) Its Power; (5). Its- material;(6) Architecture; (7) Rate of its Growth; (8) Results of its Growth; (9) The Age of Deltas; (10) Number and Geographical Distribution of Deltas; (11) Classification of Deltas. The second part treats of the various causes of the origin of deltas, the causes and con ditions of their formation, in which the author discusses; various processes of great geological interest. Three: sheets of maps accompany this most important paper, showing, among other points, the various deltas of the world.