Page:Science vol. 5.djvu/154

 tion on lithology, not from fault to be found with the treatment of the subject, but because lithologj- has now beiKime too seiioua a study to be treated in so poropreased a form. The student who uses this book witliout previous ac- quaintance with the rock-forming minerals tlint are here briefly deacrilied cannot obtain from the forty-six pages given to this section the knowledge that they are intended to give ; un- less, indeed, there is so liberal a supplement of peraonal instruction as to make the text practi- cally unnecessary. We are familiar nowadays vritb the I'eaction against the mere verba! teach- ing of physics and chemistry, zoology and botany. The same spirit of reform should exclude brief treatment of lilhology from an elementary book on physical geology. And, if the student protests that he wisiies to gain at least a superficial knowledge of lithoiogj-, let the teacher couhdently assure him that there is no such thiug, but onlj' a superScial igno- rance. Better admit full ignorance than pre- tend to scanty knowledge, and use the space in the book and the time that would be given to it for fuller discussion of other subjects. The open admission of the author's own lack of expertness in modern lithology. by his accept- ance of a chai>ter on the igneous rocks fi-om 'professor Bonney, is evidence enough that the section in question should not have been in- serted in a book of this title.

'I'he rest of the work is more satisfactory, because the elements of the subjects that it professes to teach can really he learned from it. It is chnracteriBtically British in fact and example, although some illustrations are taken IVom other countries. Its figures are hardly so good as they should he in this day of dry-plate photographs and easy reproduction of pen- and-ink diagrams. The chapter on earthquakes needs a good revision, and a terminology might he improved that allows such expressions as ' mass or weight,' ' ridge or mesa,' using these words apparently as sjuonjmea. But, as a whole, Uie book gives brief, correct, and well- arranged mention of the more salient geologi- cal facts and theories, under the headings of 'change by internal causes ; ' 'surface agencies, destructive and constructive ; ' ' petrology and physiographic geology.' The description of the elfects of faulting ia exceptionally full ; and unconformity, overlap, and overstep receive moi'e than the usual share of attention. Under fluviatile agencies, Powell's expression, ' base level of erosion,' is accepted as the most fitting to describe this important and commonly neg- lected plane of reference ; and, after definition aud illustration, the author pertinently adds.

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that it is mainly because the early advocates of river-erosion neglected to insist on the con- trol which elevation or depression exercised on river-action, that many obaerveis have been unable to believe that rivers have had any sig- nificant share in the excavation of their valleys. There is to our mind an unnecessary scepti- cism as to the subglacial origin of bowlder-clay. The snjaU and now old glaciers, which have long ago swept their beds so clean, aflbrd only im- perfect illubtration of what went on beneath the ice-sheet just after its conquest of a land covered with the waste of secular disintegra- tion ; and there is nothing inconsistent in the belief that till was accumulated at one place, while moderate-sized lake-basins were exca- vated at another, as Geikie uuil Holland have fully shown. The localities selected for illus- tration are so largely English, that the book would require re-making to prepare it for Amer- ican schools. We wish that some of our geol- ogists who are broadly acquainted with the country east and west might undertake the

��At this time, when the interest in micro- scopical petrography is so steadily on the iitf ci'eaae, the neei.1 of a concise, accurate, anl' recent text-hook on the subject is daily b»- coming more apparent. That such a one doe» not exist in English is to be much r^retted^ but this very fact will cauae information re- gaitUng an admirable one, whioli has just appeaixMl in Germany, to prove all the more acceptable to geological students. Dr. Hus- sak's liook is short and elementary j but it contains the results, even the most recent, which have thus far been attained bv the many workers in microscopical mineralogy and lith ology. Slated in a clear manner.

"The first part treats of methods — optii chemical, and mechanical — which are n< applied to the study of rock- constituents, well as the general morphological properties-' which characterize them. Part second con- sists of a tabular arrangement of all the rock- forming minerals, with their characteristic microscopic appearance, chemical reactions, associations, decomposition products, and all other peculiarities which might serve in their accurate diagnosis, arranged in parallel col- umns. This is all given in a very small space ; but the copious and excellent references furnish

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