250 01 I>GEOLOGY PART n 



system) will remain conspicuous by their brightness. A 

 thin plate of quart. :his separation of the ; 



and crystalline parts of a rock even more s. 

 It is placed under the stage, and the' Nicol ] 

 so adjusted with reference to it that the field of the 

 microscope appears uniformly violet. i f portion 



of any rock placed on the stage will allow the violet light 

 to pass through unchanged, but the crystalline portions 

 will show other prismatic colours. The object should be 

 rotated in the field and the eye kept steadily fixed upon 

 one portion of the slide at a time, so that any change 

 may be observed. 



It would be far beyond the compass of this little 

 handbook to enter into the details of the microscopic 

 examination of rocks. The student who desires to 

 pursue the subject further will find the needful assistance 

 in the works quoted below. 1 For his satisfaction in the 



1 Sorby " On the Microscopic Structure of Crystals, indicating 

 the Origin of Minerals and Rocks," Quart. Journ. Geol. Soc. xiv., 

 453 ; Zirkel's Mikroskopische Beschaffenheit der Mineralien un<f 

 Gesteine (Leipzig, 1873) ; Rosenbusch's Mikroskopische Physio- 

 graphic der Mineralien und Gesteine, 2 vols., 3d edit. 1892-96. An 

 abridgment of this work in English has been made by Prof. 

 Iddings, and is published by Macmillan and Co. ; while the Petro- 

 graphical Tables have been translated by I >i. I . II. Hatch, and are 

 separately published by Swan, Sonnenschein, and Co. forming a 

 useful handbook. Other works are Rosenbusch's Element e der 

 Gesteinslehre, 1898 ; A. Marker's Petrology for Students (Cambridge 

 Natural Science Manuals); Rutley's The St udy of Rocks ( Longmans); 

 Cole's Aids in Practical Geology (Griffin and Co.), 3d edit. 1898; 

 Hatch's Introduction to Petrology Igneous Rocks (Swan, Sonnens- 

 chein, and Co.) An admirable little pamphlet on the use of his 

 improved microscope has been prepared by Mr. A. Dick, and may 

 be had of the makers of the instrument, Swift and Son. 



