Der 
ve 
- GEOGNOSY.  —— {Boox I, 
This may be done with any basalt. A freely swinging magnetic — 
needle is of service, as by its attraction or repulsion, it affords a 
delicate test for the presence of even a small quantity of magnetic 
iron, abe ay 4 
§ IV.—Minute or Microscopie Characters of Rocks. 
No department of Geology has been more advanced in recent years 
than Lithology, and this has been mainly due to the introduction of 
the microscope as an instrument for investigating minute internal 
structure. As far back as the year 1827, a method of making thin - 
transparent sections of fossil wood, and mounting them on glass with 
Canada balsam, had been devised by William Nicol of Edinburgh, and 
was employed by Henry Witham in an investigation of the History 
of Fossil Vegetables. 
It was not, however, until 1856 that Mr. H. C. Sorby, applying 
this method to the investigation of minerals and rocks, showed 
how many and important were the geological questions on which it 
was calculated to shed light.” Reference will be made in subsequent 

pages to the remarkable results then announced by him. To the — 
publication of his memoir the subsequent rapid development of 
microscopic research among rocks may be distinctly traced. This 
branch of inquiry has been prosecuted more particularly in Germany, 
but the microscopic method of analysis is now in use in ever 
country where attention is paid to the history of rocks.® . 
In § vii. p. 182, information is given regarding the preparation of 
sections of rocks for microscopical examination, the methods of pro- 
cedure in the practice of this part of geological research and some 
of the terms employed in the following pages. 
1. Microscopic Elements of Rocks. 
Rocks when examined in thin sections with the microscope are 
found to be composed of or to contain various elements, of which the 
more important are, 1st, crystals, or crystalline substances; 2nd, glass; 
ord, crystallites; 4th, detritus. 
‘! Small 4to, Edinburgh, 1831. This work, though dedicated to Nicol, does not 
distinctly recognize him as the actual inventor of the process of slicing mineral substances 
for microscopic investigation. All that was original in Witham’s researches he owed 
either directly or indirectly to Nicol. ' 
* Brit. Assoc. 1856, Sect., p. 78. Quart. Journ. Geol. Soc. xiv. 1858. 
* Among the best text-books on this subject the following may be mentioned :— 
Mikroskopische Beschaffenheit der Mineralien und Gesteine, F. Zirkel, 1 yol. 1873. 
Milroskopische Physiographie der Mineralien und Gesteine, H. Rosenbusch, 2 vols. 1873-7. 
Elemente der Petrographie, Von Lasaulx, 1875. Minéralogie micrographique: roches 
éruptives frangaises, Fouqué et Michel-Lévy, 2 vols. 4to. Paris, 1879. | Microscopical 
Petrography, Zirkel, being vol. vi. of the Geol. Explor. of 40th Parallel, Washington, 
1876. ‘The volumes for the last ten or fifteen years of the Quarterly Journal of the 
Geological Society, Geological Magazine, Neues Jahrbuch fiir M ineralogie, &e., Zeitschrift der 
Deutschen Geologischen Gesellschaft, Bulletin de la Société géologique de France, Jahrbuch 

