L. V. Pirsson — Rise of Petrology as a Science. 225 



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 crys- 

 tallographic 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 print- 

 ing 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 pre- 

 pared by the microtome. This is no small achievement. 



The first thin sections appear to have been prepared in 

 1828 by William Mcol 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 

 hot 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 suffi- 

 ciently thin. This method was used by others for the 

 study of fossil woods, coal, etc., but it was not applied to 



Am . Jour. Scl— Fourth Series, Vol. XLVI. No. 271.— July, 1918. 

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