1888.] MICROSCOPICAL JOURNAL. ' 71 



curate method of identification of minerals wiien they could be obtained in 

 crystals. But with the greater number of occurrences of massive rocks such 

 methods of study were found impracticable, owing to the fineness of the rock- 

 texture. * It is, therefore, no matter for surprise that the finely crystalline rocks 

 were supposed to be entirely made up of some one mineral, and were given 

 names having that significance. It was not until 1815 that Cordier showed 

 the complex character of basalt by a separation of its constituents by means 

 of differences in specific gravity. 



The early attempts to subject rocks to a microscopical examination were 

 failures, because they only made use of reflected light. Transmitted po- 

 larized light was employed by Sir. David Brewster in 1816 to study the inclu- 

 sions in topaz, beryl, and other very transparent minerals. The experiments 

 of Wm. Nicol with petrified wood, fourteen years later, were most important. 

 They embrace the idea which, later, caused the birth and development of 

 microscopic petrography. Nicol made a thin, transparent section of petrified 

 wood by attaching a small fragment to a piece of wood by means of cement. 

 By the use of a grindstone the greater part of the chip was ground away. It 

 was then polished and attached by Canada balsam to a small plate of glass, 

 after which the process of grinding and polishing was applied to the other side, 

 until the chip was thin enough to be transparent. It was then ready to be 

 examined with the microscope. It is, perhaps, a little surprising that the im- 

 portance of this method was not at once appreciated and applied to the study 

 of rocks, inasmuch as the experiments of Nicol were made in the related 

 science of palaeobotany. 



It was not until twenty years later (1850), however, that the application 

 was made, when H. Clifton Sorby, of Sheffield, England, examined a speci- 

 men of calcareous grit, in thin section, under the microscope. And even 

 then neither Sorbv nor his contemporaries appreciated the value of the inven- 

 tion. Twelve years more passed before the method was made applicable to 

 the purposes of petrography through the efforts of Ferdinand Zirkel, now 

 professor of petrography at Leipsig. During Sorb3''s travels in Germany, in 

 1862, he became acquainted with Zirkel, then a student of mining at Bonn, 

 to whom, in frequent excursions that they made together, he communicated 

 the results of his studies with the microscope, and imbued the young student 

 with his own enthusiasm. 



During the winter of 1862-3 Zirkel began a careful study of minerals in 

 the thin section. It was soon found that the minerals exhibited under the 

 microscope characters altogether different from those noticed in the hand 

 specimen. The most delicate of these were those which depended upon the 

 action of the crystal upon polarized light. In order to appreciate some of 

 these, it will be necessary briefly to describe the modern petrographical micro- 

 scope, the essential features of which were embodied in the instrument which 

 Zirkel vised in his preliminary studies during the years 1S62-3. 



The common form of petrographical microscope is similar, in general con- 

 struction, to that in use for biological study. The stage is so constructed as 

 to be capable of revolution about a vertical axis, and allows the measurement of 

 plane angles by being furnished with a peripheral graduation. Below the stage 

 is adjusted in the axis of the instrument a Nicol prism for producing polar- 

 ized light. This prism is called the polarizer. A second Nicol prism, 

 called the analyzer, may be introduced above the objective. 



The sections of a given mineral will, in general, show by their form in 

 what direction they are cut, the external planes of the crystal being repre- 

 sented by the straight lines which limit the section (unless, owing to cleav- 

 age, the section has broken in grinding) . In like manner, the cleavage planes 

 will be shown in the section as series of parallel cracks, the nearness of which 

 to each other is an index of the perfection of cleavage. 



