THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



YoL. YII. 



Washington, D. C, Makch, 1886. 



No. 3. 



The Microscopical Study of Rocks.* 



BY J. S. DILLER, ASST. GEOLOGIST, 

 U. S. GEOL. SURVEY. 



The speaker began by calling at- 

 tention to the extensive use of the 

 mici'oscope in the study of rocks, and 

 briefly i-eviewed the history of its ap- 

 plication in the development of mod- 

 ern petrography. 



The earlier observations pertained 

 chiefly to precious stones and their 

 inclusions, but M. F. Ledermiiller 

 and H. Baker soon after the middle 

 of the eighteenth century called atten- 

 tion to the structure and genesis of 

 crystals. In 1780 C. A. Gerhard 

 studied mineral sections under the 

 microscope in discovering the struc- 

 ture of chiastolite, but it is important 

 to note that these sections were 

 studied only in reflected light. The 

 preparation and study of the first 

 really thin sections in transmitted 

 light was eflected by William Nicol. 

 who, in 1S31, discovered the calcite 

 prism that bears his name. The in- 

 vestigations of Nicol, in connection 

 with those of Sir David Brewster 

 and Sir Humphrey Davy, concerning 

 the optical properties of minerals, 

 were of prime importance in the de- 

 velopment of petrographic research. 



There are two ways in which a 

 rock may be prepared for microscopi- 

 cal investigation. It may be pulver- 

 ized or it may be sliced. As the 

 preparation of thin slices of rocks is 

 attended with considerable difficulty 

 and their pulverization is easily ac- 

 complished, the earliest microscopical 

 observations of rocks were made upon 



* Abstract of a communication to the Washington 

 Microscopical Society, Feb. 9th, 1886. 



their powder. Before the close of 

 the eighteenth century Dolomieu and 

 others had used the microscope in 

 studying pulverized rocks. These 

 observations were soon followed by 

 those of Zincken in Germany, but it 

 was not until many years later that 

 the microscope was S3'stematically em- 

 ployed in petrographic research . 

 This important application was made 

 by Henry Clifton Sorbv, who was the 

 first to fully appreciate the value of the 

 microscope in studying rocks. He 

 published in 1858 in the ^u art . 

 Jour 71. Geol. Soc.., London, a paper 

 "• On the microscopical structure of 

 crystals, indicating the origin of min- 

 erals and rocks,' and may be con- 

 sidered the chief initiator of modern 

 petrographic methods. The seed 

 sown by Sorby in England soon bore 

 abundant fruit in Germany, for in 

 1863 we find F. Zirkel publishing the 

 first of a number of books which 

 mark the beginning of an epoch in 

 geologic investigations. A host of 

 enthusiastic Germans -who were well 

 prepared for the work then took it up, 

 and micro-petrography developed 

 with wonderful rapidity. To Prof. 

 H. Rosenbusch,who in 1873 published 

 the first volume of his Mikroskopische 

 Phvsiographie (der petrographisch 

 wichtigen mineralien) , and a second 

 volume (der Massigen Gesteine) in 

 1877, the greater portion of this rapid 

 development is due. These masterly 

 works have laid open such fertile 

 fields for investigation, and inspired 

 so much enthusiasin into their cul- 

 tivation that it is not surprising to find 

 petrography one of the most progres- 

 sive of all the branches of science. 



