Page:Popular Science Monthly Volume 31.djvu/143

Rh inorganic and those of organic nature are divided into a descriptive and a philosophical group. In writing the six succeeding essays it was the author's design to bring together in a concise form the facts and the reasonings from which are deduced what he regards as the principia of geogeny, geognosy, and mineralogy. The chemistry of the atmosphere, and the relations of the earth's aerial envelope both to outer space and to the gases condensed, and the waters precipitated on the surface of the globe, are set forth in the third and fourth essays, as a preliminary to the study of rock-masses. In the next three essays the genesis and the geognostic relations of the various crystalline rocks are considered; and finally the decay of these, which has determined their present surface-outlines, and has given rise to the materials of the uncrystalline sedimentary strata. In the fifth essay, Professor Hunt attempts to show the defects of the several other proposed explanations of the origin of the crystalline rocks, and sets forth his crenitic hypothesis, according to which they have been derived—for the most part indirectly and by aqueous solution—from a single primary plutonic mass. These and other related points are more fully discussed in the sixth essay.

A system of classification for the mineral species composing the earth's crust, which should consider their physical characters in connection with their chemical composition, and the mode of formation of mineral species, has seemed to the author for many years to be a desideratum. What he believes to be a natural classification of the native silicates is included in this volume, and is followed by an outline of the system as applied to all other native minerals. Regarding the silicates as a natural order, Professor Hunt divides them into three sub-orders: those without alumina, which he calls protosilicates; those with alumina and containing combined protoxides, protopersilicates; and those with alumina, but no protoxides, persilicates. These suborders are divided each into five tribes, according to distinctions of structure, hardness, and density. Each tribe is made up directly of species. The silicates are included in the same class with the oxidates, titanates, niobates, tantalates, tungstates, molybdates, chromates, vanadates, antimonates, arsenates, phosphates, nitrates, sulphates, borates, carbonates, and oxalates. This class is numbered II; Class I comprises the metallates; Class III, the haloidates; and Class IV, the pyricaustates, or combustible species. The author has in preparation a treatise on mineralogy which will be a complete presentation of this system.

The next essay deals with the geological history of pre-Cambrian rocks, both in North America and in Europe, and is mainly a condensation from the account given in the author's volume on "Azoic Rocks." Intimately connected with this subject is the history of serpentines, which is sketched in the following essay. The various opinions as to the geognostical relations of serpentines which have been held by different writers are shown, and the author's reasons for maintaining their aqueous origin are given. In the concluding essay, the question of the Taconic rocks is discussed at some length. The eleven essays which are collected in this volume are papers which have been presented to American and British learned societies, and have been published in their transactions and in scientific journals. Changes have been made occasionally in revision, but all additions of importance are inclosed in brackets.

course of instruction pursued at Fort Meyer has been enlarged and otherwise improved, and now provides for theoretical and practical instruction in the duties required of the Signal Corps in time of war. Lectures by professors of meteorology have been provided for; and a course of instruction in military surveying, field-sketching, and topographical drawing has been added. A text-book of meteorology has been prepared by Professor William Ferrel, and forms the second part of the present report. Professor Cleveland Abbe has in preparation a treatise on the theory of instruments used in meteorology. Translations of papers on temperatures and storms by Ragone and Wild are appended; and translations have been made of