l888.] NEW-YORK MICROSCOPICAL SOCIETY, 143 



more than one respect, revolutionized our old-fashioned concep- 

 tions of geological research. 



A number of very excellent works have been published by 

 eminent specialists, notably Germans, such as Rosenbusch, 

 Zirkel and others ; but, like most works of the kind, they are 

 too dry and technical to attract or to satisfy the amateur, who 

 wants something explicit and popular to make a study pleasant. 

 These books take too much for granted, and presuppose knowl- 

 edge which not one in a thousand, of even well-educated per- 

 sons, is likely to possess. They invariably assume that the 

 reader has a thorough knowledge of crystallography, a subject 

 full of complexities and difficulties, and abounding in puzzles 

 sufficient to try the patience of a Job. They also expect the 

 student to be well acquainted with optics, especially that most 

 difficult and exasperating branch, which relates to the polariza- 

 tion of light, and the chromatic phenomena presented by crystals 

 under crossed Nicol-prisms. Of all subjects difficult to under- 

 stand, this last is the worst, and the explanation of it generally 

 given in books leaves one more bewildered than instructed. It 

 took me over two years to penetrate the mystery, and now I 

 find that I can explain in a quarter of an hour what has taken 

 me so long to learn. 



What is wanted is a popular work, consisting of two parts ; the 

 first to give a clear and intelligible account of crystallography, 

 optics and other necessary or desirable preliminary information; 

 the second, to treat of the structure of rocks, the whole to be 

 written in as light and attractive a manner as the subject per- 

 mits, and to take as little for granted as possible. I am perfectly 

 aware that such a book would necessarily lack in thoroughness 

 and completeness, but to the beginner it would be of incalculable 

 value. 



Rutley's work, " The Study of Rocks, an Elementary Text Book 

 of Petrology," contains much useful information, and is a very 

 good book for a beginner; but what it says on crystallography 

 and the phenomena of polarization is altogether too short and 

 fragmentary. 



But let those of you, who have a mind to enter the field of 

 microscopical petrology, who have a general desire to join 

 the ranks of workers in this very attractive and interesting de- 



