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Petrographic Analysis

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PETROGRAPHIC ANALYSIS.

The following methods of examination have been used by the Office of Public Roads of the U. S. Agri cultural Department, and are described by Mr. Edwin C. E. Lord in Bulletin No. 31, August, 1907.

"Upon receipt of the rock sample, which, accord ing to the specification of this Office, should weigh not less than 30 pounds and be collected with care to repre sent as nearly as possible an average of the whole exposure, it is examined in a general way to determine the proper method of analysis.

"Rocks consisting essentially of the carbonates of lime and magnesia (limestones, dolomites, etc.), as well as fine-grained shales and unconsolidated sedi mentary deposits, such as clays, sands, gravels, etc., are analyzed chemically when necessary, whereas all other materials are prepared for microscopic exami nation or determined macroscopically.

' "The mineral composition of a rock may, under favorable conditions, be estimated with considerable accuracy by a macroscopic examination, yet for exact quantitative results the aid of a polarizing microscope and transparent thin sections of the rock are essential.

Macroscopic Method. "The macroscopic form of analysis can be applied only to coarse-grained rock, in which the various mineral components are easily detected with the unaided eye. The approximate volumetric relations of these minerals may be deter mined by preparing a smooth surface of the rock sample and covering it with a transparent celluloid scale divided into too equal square areas and estimat ing the minerals present from the number of areas . covered by each mineral. Any properly graduated scale can be used, but a transparent one is preferable." Microscopic Methods. to the large amount of material received in this laboratory it has been found necessary to perfect a more rapid method of quantitative analysis than any hitherto described.

"The laboratory is equipped with an exceptionally good petrographic microscope of the latest Fuess model, which, beside the usual attachments, is provided with a revolving analyzer in the tube to aid in the deter mination of very low double refracting minerals, and a Schwarzmann scale for the measurement of optical axial angles.

"Another important accessory is a detachable screw-micrometer, movable in the focal plane of the ocular by means of a drum screw, which, with the most powerful objective (one-twelfth-inch oil immer sion), records a drum-interval of 0.00004 mm. The measuring apparatus devised by Mr. L. W. Page and used for the mineral determinations consists of an ordinary fixed eyepiece having a square field divided into 100 quadratic areas. With the aid of this cross line field, each square of which is one one-hundredth of the whole field, the relative proportions, expressed in per cent, of the minerals occupying the field can be readily determined by simply noting the number of squares covered by each mineral in turn. Averages derived from numerous examinations of this kind in various parts of the section indicate the percentage of the different minerals constituting the rock itself." "Experience has shown that with a large majority of rock samples twenty determinations, using a magni fication of 52 diameters, give very satisfactory results. In the case of extremely fine-grained rocks, however, it is best to use a three-quarter-inch objective lens which enlarges 105 diameters when combined with the eyepiece micrometer.

"With rocks having an average grain exceeding 5 mm., or those varying greatly in texture, as in the case of porphyritic and schistose varieties, it is in some instances well to employ a two-inch objective in combination with an ocular prepared in the . same manner as that just described, but divided into only 25 , square areas and magnifying 3o diameters. In the case of these exceptionally coarse-grained rocks, two or more thin sections of the same sample are examined before reliable results can be obtained. " The physical properties of stone for road-building are commonly tested by determining the percentage of wear, using the Deval abrasion apparatus, and the cementation properties by the use of the Page cementa tion test. Tests are also sometimes made of the crush ing strength, resistance to impact, and resistance to abrasion by grinding, and, in some instances, the specific gravity and absorption of the rock are deter mined.

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