Forbes — The Microscope in Geology. 511 



acquiring a practical knowledge of the more common forms with 

 which he ought to be acquainted, or is likely to meet with, as well 

 as to form a useful aid to teachers in public or private schools who 

 may desire to impart to their pupils a knowledge of the elements of 

 geology and mineralogy. The geological series comprises two 

 volumes, each containing twelve specimens, neatly arranged and 

 named with their geological position and localities, one of British 

 rocks, the other of British fossils. The mineralogical series is also 

 contained in two cases, one comprising the earthy minerals from 

 which many important and useful preparations are extracted for use 

 in the arts and manufactures ; the other contains the mineral ores 

 from which the metals in general use are extracted. Appended to 

 each volume is a brief description of the different specimens, as well 

 as some general and introductory information useful to those com- 

 mencing the study for the first time. Thus, under " British rocks," 

 the classification and the chief subdivisions are given ; and under 

 " Metallic minerals," the priucipal characters of minerals and terms 

 used in mineralogy are noticed. Each volume is complete in itself, 

 and the cheap price at which they are produced ought to guarantee 

 for them an extensive circulation. 



III. — The Microscope in Geology, 



UNDEE this title we find a very interesting article in the 

 "Popular Science Eeview" (for October, p. 355), by David 

 Forbes, F.R.S., which so entirely relates to our own special study, 

 that we trust we shall be excused for giving an unusually long 

 notice of it : we only regret being unable to present our readers 

 with copies of the two beautiful plates illustrating the •' Microsco- 

 pical structure of Eocks," which accompanies it. After pointing out 

 the indispensable nature of microscopic inquiry, in order to solve the 

 true character of rock-masses, the author proceeds to describe the 

 best method of preparing sections of rocks of the needful trans- 

 parency for microscopic investigation. 



" The examination," says Mr. Forbes, " of such a rock-section 

 enables a mineralogical analysis to be made, even of the most com- 

 pact and apparently homogeneous rock, and generally leads to the 

 discovery of other mineral constituents previously unsuspected, from 

 their being invisible to the eye, and also, as Sorby has observed, 

 allows those minerals, formed at the time of solidification of the 

 rock, to be distinguished from such as are the products of subsequent 

 alteration. 



Arranging rock species according to their structure, it will be 

 found that most rocks fall naturally into one or other of two great 

 classes — 



I. Pkimary or Eruptive Eocks ; 

 II. Secondary or Sedimentary Eocks ; 



and it will be seen that the microscope is of special value when 

 applied in cases where the external appearance renders it doubtful 

 as to which of these classes a rock may pertain. 



