GEIKIE'S TEXT-BOOK OF GEOLOGY.* THE favourite scientific text-books of one
generation, before they can answer the requirements of another, need to be "greatly enlarged," to los " recast," or to be " almost entirely rewritten." But a time must come when the most strenuous efforts of author and editor fail to bring back the waning popu- larity of a once-valued manual. Henceforth it may take an assured place among books of reference, but it remains no longer a constant guide and companion in field and laboratory and study. Fortunate it is for students and teachers alike when a worthy successor can be found,—a successor as wide as its predecessor in range, as full in research, as happy in suggestion, as cogent in argument, and as attractive in method and in form.
It would not be an adequate expression of the high opinion. we have formed of the merits of Dr. Geikie's Text-book of Geology, were we to say that it will take its place as the new class-book of this science; a work of much lower quality might have done this. But the author possesses something more than the learning and sound judgment necessary for the task which he undertook. To discover, to select, and to arrange materials constitute but the necessary foundation for his work ; firmness of outline and due proportion in the different parts are as much needed, to secure a satisfactory result. But the author of the text-book before us may claim in an unusual degree other merits besides these, namely, the merits of originality of view and picturesqueness of expression. In a book for students' use, the latter quality, though of obvious importance, will not appeal only to beginners.
Perhaps the illustrations, in a volume having the scope of a natural-science text-book, demand a few special words of reference, before the scheme of the whole work is discussed. We may at once say that the illustrations--437 in number—are not merely well chosen, but in very many cases something more than adequate. Naturally, the frontispiece takes our attention at once. It represents the grand plateau and cafions of the Colorado, and a very fine drawing it is, full, indeed, of laborious detail, but every touch telling, and the complex perspective being managed with consummate skill. Dr. Geikie informs us that it is a reduction of a drawing by Mr. W. H. Holmes, and was furnished by Captain Dutton, of the United States Geological Survey. When we study this drawing, and then turn to the two diagrams by the same draughtsman on p. 923, we are able to convince ourselves that Dr. Geikie has not said a word too much in praise of Mr. Holmes's powers as a delineator of the geological structure and scenery of Western America. His pictures are certainly most remarkable examples of the union of " artistic effectiveness with almost diagrammatic geological distinctness and accuracy." But the author's own drawings, particularly those of "scenic geology," present the same elements of picturesqueness and accuracy, for we presume we are right in attributing to Dr. Geikie's own pencil such drawings as those reproduced on pages 433, 555, 642, &c. Many original sketches of another class, but from the same source, are to be- found in several parts of the volume, although they are chiefly gathered into the chapter which treats of the microscopic characters of rocks ; we refer to the magnified drawings from thin rock-sections.
It is now time to tell our readers something of the contents of this text-book, and something of the order which has been followed in its construction. The contents are divided into seven sections, or "books," of unequal length, varying, indeed, from twenty pages to two hundred and fifty. The seven books are further divided, more or less elaborately, into parts, sections, subsections, paragraphs, &c, according to the range and char- acter of the subjects under treatment. Book i. is headed," The Cosmical Aspects of Geology," a title which sufficiently explains itself. In Book ii. are discussed the materials of the earth's substance, including, firstly, the atmosphere and the ocean as its envelopes, and the solid globe, its temperature and age ; then, secondly, an account of the composition of the earth's crust, or "minerals and rocks." " Geognosy " is the alternative title of this second book. So considerable is the im- portance of the manner in which rocks and minerals are first studied, that we may pause to note the order which Dr. Geikie here follows. Beginning with the general chemical constitution of the crust, and with rock-forming minerals, he passes on to the study of the general or macroscopic characters of rocks, and then to their minute or microscopic characters; afterwards
* nal-book of Geology. By Archibald Geikie, LL.D., F.R.S., DirectonGeneral of the Geological Survey. London : Macmillan and 0o. 1852. comes classification. Here Dr. Geikie abandons arrangements drawn even in part from processes of formation, real or sups posed, and relies on the broad distinction subsisting between crystalline and fragmental rocks. Still, however, considerable violence is thus done to the term " crystalline," since it is made to embrace materials which are vitreous, and therefore certainly non-crystalline. Of the further divisions, three are included under crystalline rocks, and four under the fragmental, the former being " stratified," " foliated," or "massive ;" the latter being " psammites," " pelites,; " tuffs " or " fragmental rocks of organic origin." The stratified rocks include chemical de- posits, such as limestones, dolomites, and sinters ; the foliated or schistose rocks correspond mainly to the metamorphic rocks, while the massive rocks coincide pretty nearly with the old division of igneous rocks. It is in his treatment of these massive rocks, the granites, felsites, syenites, tmchytee, phonolites, &c., that we find a good example of Dr. Geikie's accurate and recent knowledge, as well as of his grasp of view. Modern petrology has of late made such a large use of new methods for the microscopical and chemical study of rock-constituents, that the danger of looking at rocks from an exclusively chemical and mineralogical point of view has been much increased. Dr. Geikie avoids such an error, while according a due amount of value to characters derived from exact chemical and mineralo- gical determinations. Here we may venture to point out that our author has adopted (page 193), from MM. Fouqu6 and Michel Ldvy, the common Continental mistake of attributing to M. Thoulet the ingenious process of separating mixed minerals from one another by means of a dense liquid, in which some species float, while others sink ; the process had been in use in England by means of Sonstadt's liquid, some time before M. Thoulet published his experimenti, in which he recommended the ordinary solution of mercuric iodide in potassium iodide.
We cannot afford to linger long over the fascinating contents of Book iii., a most important section of the work, treating of dynamical geology, and extending over nearly 300 pages. Many of our readers who might not care to enter upon a study of palaeontological geology in Book v. or of stratigraphical geology in Book vi., would be glad to know where they might turn to find the story of the changes which are going on beneath the earth, and of those which are in progress upon. its surface. Under the former heading of " Hypogene Action," volcanoes, earthquakes, secular subsidences and upheavals, and under- ground changes of texture, structure and composition in rocks, are considered. " Epigene," or surface action, includes the work of air, of water, and of life. Such important topics are here dwelt on as rain and the weathering of rocks ; springs, rivers, lakes, glaciers, oceans, and tides, as well as the formation of soils. It is hard to say which we admire most, the skill with which so much inforMation is compressed into so small a space, the reticence which has excluded topics and details of secondary moment, or the clearness which marks each description, calcula- tion, or inference.
Of Books v. and vii. we have left ourselves no room to speak. They treat respectively of " Geotectonic Geology," that is, the architecture of the earth's crust, and of " Physiographical Geology ;" but we must, in passing, commend the exactitude with which the many subjects included ander " Structural Geology " are handled.
Books v. and vi., treating largely, as both of them do, of fossil remains, necessarily overlap. But it is remarkable to observe how little repetition there is here in the treatment of these two subjects of palaeontological and stratigraphical geology. It is worthy of note that Dr. Geikie expresses himself cautiously with regard to eozoon, and to several other subjects where there is room for genuine doubt. His attitude with regard to Sedgwick and Murchison is less pronounced than we expected. An index of subjects occupying thirty-six pages, with double columns, and an index of authorities of ten columns, are not the least meritorious features of this text-book. Some of the more definitely chemical statements of these pages need slight revision. We note a few which caught the eye in turning over half the pages of Book ii. The pro- portion of carbonic-acid gas in the atmosphere is over- stated on p. 31. For some years past, more accurate analyses have tended to show that the accepted figure, four volumes in 10,000, should be reduced to three parts. On page 57 the im- pression given that such a mineral as cryolite contains mere traces of fluorine should have been avoided. In tabulating what is called the average composition of felspars, it is a mis-
take to carry the decimal figures to the second place. On page -85, and elsewhere in this and other sections of the book, we find no express reference to the occurrence and functions of iron pyrites in a finely-divided condition as a rock constituent. And we have failed to discover any recognition of the relation of the sulphur to the other constituents of carbonaceous rocks,— indeed, sulphur is not named as a constituent of coal in the table of analyses given on page 172. If we point out these few small defects as specimens of a class, it is in no spirit of fault- finding, but because we should be glad to remove any spot, how- .ever insignificant, from so fair a picture.
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