Geology and Mineralogy . 145 



direction — from the general to the particular. A single fossil 

 in most cases is sufficient to determine the time (Paleozoic, Meso- 

 zoic or Cenozoic) to which the formation belongs. The times 

 may be divided into Eras, the Cambrian, Silurian, Carboniferous, 

 etc. Refinement in general usage has proceeded far enough to 

 permit the recognition of two-fold or three-fold divisions of the 

 eras, and these may be designated by prefexing eo, rneso and neo 

 to the name of the era, and may be called Periods ; thus Eocam- 

 brian is the name of the period of the Olenellus fauna, Mesocam- 

 brian, is the period of the Paradoxides fauna, Neocambrian the 

 period of Dicellocephalus fauna — and so on for each of the other 

 eras. 



Thus we would say of the Chemung, the Famennian, the 

 Ilfracombe formations, each belongs to the Neodevonian period. 

 The particular time value of each of these formations in a general 

 time-scale we do not yet know with precision. Although the 

 perfecting of knowledge will undoubtedly enable future geolo- 

 gists to define epochs and even hemerce, in the general geological 

 time-scale, our present knowledge of the succession of varieties 

 and species is not sufficiently accurate to make their definition 

 practicable for any extended region. While, therefore, we know 

 the order of succession of faunas in local formations we may not 

 be able to correlate our faunas with greater precision than to 

 define periods for the standard time-scale. Common usage signi- 

 fies that already this can be done so far as to recognize three- 

 fold, or in some cases only two-fold subdivisions of the following 

 eras, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, 

 Triassic, Jurassic, Cretaceous and Tertiary. (Author's abstract.) 



h. s. w. 



2. Le Tormaline del Granito Elbano di Giovanni D'Achiardi, 

 parte prima 8°, pp. 82, one plate, Pisa, 1893 (T. Nisti'i). — Prof. 

 D'Achiardi states in his preface that the purpose of making a 

 mineralogical study of the tourmalines of Elba was suggested by 

 the immense richness of the material in the museum at Pisa and 

 in the present part he gives the bibliography relating to the min- 

 eral and " treats of the morphology of the Elban tourmalines de- 

 scribing the crystal form in relation to their multiple varieties." 



l. v. p. 



3. Anleitang zur Krystallberechnung von Dr. Benno Hecht. 

 8°, pp. 76. Leipzig, 1893 (Johann Barth). — This small volume 

 gives in highly condensed shape the mathematical relations and 

 formulae necessary lor the calculation of crystal forms and ele- 

 ments. The method employed is empirical and given from a 

 purely mathematical standpoint. It will be of interest to those 

 engaged in crystallographic studies. l. v. p. 



4. Cours de Miner alogie prof esse a la Faculte des Sciences de 

 Paris par Charles Friedel, Mineralogie generale. 416 pp. 8°, 

 Paris, 1893 (G. Massen). — The object of this text book, so we 

 are informed by the author in the preface, is to supply a more 

 general work and one which does not demand so much training 



Am. Jour. Scl— Third Series, Vol. XLVII, No. 278.— Feb., 1894. 

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