The beginnings of a particular branch of science are generally obscure and rooted so imperceptibly in the foundations on which it rests that it is difficult to point to any particular place in its development and say that this is the start. There are exceptions of course, like the remarkable work of Willard Gibbs in physical chemistry, and it may chance that the happy inspiration of a single worker may give such direction to methods of investigation as to open the gates into a whole new realm of research, and to thus create a separate scientific field, as happened in Radiochemistry.
This is what occurred in petrology when Sorby in England, in 1858,[[109]] pointed out the value of the microscope as an instrument of research in geologic investigations, and demonstrated that its employment in the study of thin sections of rocks would yield information of the highest value. Others beside Sorby had made use of the microscope, as pointed out by Zirkel,[[110]] but, as he indicates, no one before him had recognized its value. During the next ten years or so, however, its recognition was very slow and the papers published by Sorby himself were mainly concerned in settling very special matters.
As Williams[[111]] has suggested, the greatest service of Sorby was, perhaps, his instructing Zirkel in his ideas and methods, for the latter threw himself whole-heartedly into the study of rocks by the aid of the microscope and his discoveries stimulated other workers in this field in Germany, his native country, until the dawning science of petrology began to assume form. A further step forward was taken in 1873 in the appearance of the text-books of Zirkel[[112]] and Rosenbusch[[113]] which collated the knowledge which had been gained and furnished the investigator more precise methods of work. It is difficult for the student of to-day to realize how much had been learned in the interval and, for that matter, how much has been gained since 1873, without an inspection of these now obsolete texts. In 1863, Zirkel, who was then at the beginning of his work, said in his first paper presented to the Vienna Academy of Sciences[[114]] that if he confined himself chiefly to the structure of the rocks investigated and of their component minerals, and stated little as to what these minerals were, the reason for that was because “although the microscope serves splendidly for the investigation of the former relations, it promises very little help for the latter. Labradorite, oligoclase and orthoclase, augite and hornblende, minerals whose recognition offers the most important problems in petrography, in most cases cannot be distinguished from one another under the microscope.” How little could Zirkel have foreseen, at this time, less than forty years later, that not only could labradorite be accurately determined in a rock-section, but that in a few minutes by the making of two or three measurements on a properly selected section, its chemical composition and the crystallographic orientation of the section itself could be determined!
The Thin Section.
Before going further we may pause here a moment to consider the origin and development of the thin section, without which no progress could have been made in this field of research. When we reflect upon the matter, it seems a marvelous thing indeed that the densest, blackest rock can be made to yield a section of the ¹⁄₁₀₀₀ of an inch in thickness, so thin and transparent that fine printing can be easily read through it, and transmitting light so clearly that the most high-powered objectives of the microscope can be used to discern and study the minutest structures it presents with the same capacity that they can be employed upon sections of organic material prepared by the microtome. This is no small achievement.
The first thin sections appear to have been prepared in 1828 by William Nicol of Edinburgh, to whom we owe the prism which carries his name. He undertook the making of sections from fossil wood for the purpose of studying its structure. The method he developed was in principle the same as that employed to-day, where machinery is not used; that is, he ground a flat smooth surface upon one side of a chip of his petrified wood, then cemented this to a bit of glass plate with Canada balsam, and ground down the other side until the section was sufficiently thin. This method was used by others for the study of fossil woods, coal, etc., but it was not applied to rocks until 1850, when Sorby used it for investigating a calcareous grit. Oschatz, in Germany, also about this time independently discovered the same method. A further advance was made in melting the cement, floating off the slice, and transferring it to a suitable object-glass with cover, a process still employed by many; though most operators now cement the first prepared surface of the rock chip directly to the object-glass, and mount the section without transferring it.
Next came the use of machinery to save labor in grinding, and another step was made in the introduction of the saw, a circular disk of sheet iron whose edge was furnished with embedded diamond dust. This makes it possible to cut relatively thin slices with comparative rapidity, but the final grinding which requires experience and skill must still be done by hand. Carborundum has also largely replaced emery. The skill and technique of preparers has reached a point where sections of rocks of the desired thinness (0·001 inch), and four or five inches square have been exhibited.
The Era of Petrography.
In these earlier days of the science, as noted above, great difficulty was at first experienced in the recognition of the minerals as they were encountered in the study of rocks under the microscope. At that time the chemical composition and outward crystal form of minerals were relatively much better known than their physical and, especially, their optical properties and constants. Some beginnings in this had been made by Brewster, Nicol, and other physicists, and the mineralogists had commenced to study minerals from this viewpoint. Especially Des Cloiseaux had devoted himself to determining the optical properties of many minerals, and the writer, when a student in the laboratory of Rosenbusch in 1890, well recalls the tribute that he paid to the work of Des Cloiseaux for the aid which it had afforded him in his earlier researches in petrography.
The twenty years following the publication of the texts of Rosenbusch and Zirkel may be characterized as the era of microscopical petrography. A distinction is drawn here between the latter word and petrology, a distinction often overlooked, for petrography means literally the description of rocks, whereas petrology denotes the science of rocks. As time passed the broader and more fundamental features of rocks, especially of igneous and metamorphic rocks, in addition to their mineral constitution, were more studied and gained greater recognition, petrography gradually became a department of the larger field of petrology—the science of to-day.