METHODS OF TESTING SLATE. 45 
Third. Make compass determination of the strike of such beds. 
Fourth. Explore along that strike. 
Fifth. Explore at right angles to that strike to see if the series is repeated by fold- 
ing. (Note order of horizons in table facing p. 94. ) 
Sixth. Trench at promising localities across the strike to expose as large a series 
as possible. 
Seventh. When surface indications are favorable, make an opening large enough 
to determine angle of dip of both bed and cleavage and to obtain specimens sufficienl 
for tests given on pages 45-49. 
Eighth. Bore with diamond or steel-shot drill at 45° to cleavage dip, so thai t lie core 
will split up into elliptical pieces sufficiently larger than diameter of cote to be con- 
veniently tested. 
Ninth. Measure thickness at right angles to bedding planes on the core. 
The map of the Slatington, Pa,, slate district (PI. XVI), upon which the quarries 
are carefully located and the strike and pitch shown, is also designed for practical use. 
METHODS OF TESTING SLATE. 
Methods of testing the elasticity, absorption, fissility, and resistance of roofing 
slates have been in use for many years, and many more or less complete chemical 
analyses of slate have been published. In recent years, however, more exact methods 
of reaching these results have been devised. All such methods have here been brought 
together. If parts of one specimen, fairly representing the average quality of the 
product of any quarry or prospect, or if parts of each of a series of specimens, fairly 
representing all the different varieties and qualities there obtained, were to be sub- 
jected respectively to the tests described, such a slate or slates may be said to have 
been for all economic purposes exhaustively investigated. Several of these tests are 
of so simple a character as to be very easily applied. This list of methods is largely 
compiled from Bottinger, Fresenius, Hutchins, Jannetaz, Merriman, Reverdin and 
de la Harpe, Sorby, Umlauft, and J. F. Williams. a Although they all offer valuable 
suggestions, the most useful papers on the subject are those of Fresenius, [Jmlauft, 
and Merriman. 
Sonorousness. — One of the first and most time-honored tests of roofing slate is to 
suspend a good-sized piece of the usual thinness and tap it with some hard object. 
If it possesses the molecular structure of a slate it will yield a semi vitreous ring. A 
clay slate will be less sonorous than a mica slate, but mica slates with a large per- 
centage of chlorite and possibly a little talc will be deficient in sonorousness. It is 
because of this property that when at the quarries refuse slates are thrown upon the 
dumps the sound produced is not unlike that made by the smashing of a large 
quantity of crockery. 
Cleavability. — This test should be applied by an experienced workman. The bl< >ck 
should be freshly quarried, unfrozen, and moist. The chisel should be very thin and 
about 2 inches wide. The cost of slate is closely related to the degree of its cleava- 
bility. 
Cross fracture {"sculping"). — This is to determine the character of the "grain." 
This test should also be applied by an experienced hand to a large block several 
inches thick, with a stout chisel and a long-handled heavy mallet. Jannetaz'' pub- 
lished a method for determining with scientific precision the direction of the grain 
in slate when it is but obscurely shown on the cleavage surface. The slate is sawn 
in a direction parallel to its cleavage and one of the sawn surfaces is made exceed- 
ingly smooth and covered with an even and very thin coat of grease. The point of 
a red-hot platinum wire is applied to the slate opposite the center of the greased 
a Full titles are given in the economic Bibliography, pp. 138-145. 
b Relations entre la propagation de la chaleur et l'elasticite sonore dans les roches, 1877, p. 417. 
