PETROGRAPHIC CHARACTERS. 9 
(tensions, shears, contractions), which resulted in several systems of joints, in faults, 
slip cleavage ("false cleavage"), shear zones ("hogbacks"), and in irregular open- 
ings in which veins of quartz and calcite were formed by infiltration; and the mass. 
in some cases, was traversed by fissures penetrating to the molten zone and thus 
permitting the exudation of lava-like material which formed dikes. 
The folds into which the slate beds were bent are rarely complete, for the reason 
that their upper portions have generally been removed through various erosive pro- 
cesses—the decomposition due to atmospheric agencies, the action of streams, and, 
in places, of the continental glacier. Pis. Ill, XIII, XIV, XVII, XVIII, and 
XXV all illustrate this feature. In the Appalachian region generally the thickness 
of this eroded material was very considerable. 
PETROGRAPHIC CHARACTERS/' 
HISTORY OF RESEARCH. 
However uncertain the structural, mineralogical, and chemical features of roofing 
slates may once have seemed, these are all now well understood. Sedgwick, Sorby, 
Phillips, Tyndall, Daubree, Gosselet, Jannetaz, and Becker have studied their 
structure either in the field or the laboratory, or in both, and Sorby, Zirkel, Renard, 
and others have investigated their mineralogical composition. Summaries on the 
cleavage of slate were published by Loretz in 1880, and Harker in 1886, and on its 
petrography by Kalkowsky in 1886, Zirkel in 1894, and Rosenbusch in 1898. Chem- 
ical analyses are given in Roth's Chemical Geology (1890), and in Zirkel' s Petrography 
(1894). Hillebrand, Reade, and Holland made a number of complete analyses 
between 1899 and 1901. Van Hise, in his recent Treatise on 'Metamorphism h 
(1904), discusses the theory of metamorphism in schists and slates, and Leith, in his 
paper on rock cleavage e (1905), treats of the theory of bedding and cleavage, basing 
his conclusions more largely on the microscopic study of schists than of roofing 
slates. The bibliography on pages 138-145 shows how much has been written on the 
different aspects of the subject. It is not proposed to give here a summary of all this 
literature, but an attempt will be made to set forth, in a series of statements, quota- 
tions, and questions; the state of scientific opinion on the more important and inter- 
esting features of slate. 
Slaty cleavage may be defined simply as a rearrangement of the particles of a dep< >sit 
by lateral pressure, together with the arrangement of any new crystalline particles 
formed during and in consequence of that pressure. This arrangement of old and 
new particles is related to the directions of pressure and resistance. The older 
authorities on slaty cleavage usually define the direction of pressure as being at 90° 
to the cleavage. This definition of the direction has, however, been under discussion 
in recent years. The latest phase of opinion appears in the writings of Van Hise, 
Becker, and Leith. Van Hise rf expresses his views in these words: 
During the process of recrystallization at any given moment there is maximum shortening in the 
direction of greatest stress, maximum elongation in the direction of least stress, and shortening or 
elongation in the direction of mean stress. Consequently the shape of the modified particle may be 
that which would be produced if a plastic grain were rolled out, the sides being confined in one 
direction, but with liberty to elongate in another direction in thesame plane; oritmay be thatwhieh 
would be produced if a roundish cake of dough were flattened between two boards, and consequently 
elongated in all directions at right angles to the direction of greatest pressure. . . . The particles 
are arranged with their greater, mean, and minor diameters, corresponding to least, mean, and greatest 
pressures. In proportion as the movement involves shearing motion or scission the mineral part Lcles 
are rotated from a position in which the direction of greatest elongation is at right angles to the direc- 
tion of greatest pressure, although at any given time the mineral particles tend to develop with their 
longer axes at right angles to the maximum pressure. 
•*This chapter in its original form appeared in part 3 of Nineteenth Ann. Rept. U. S. Geol. Survey, 
in 1899, but its matter is here revised and brought up to date. 
Hlon. U. S. Geol. Survey, vol. 47. 
("Bull. U. S. Geol. Survev No. 239. 
^Treatise on metamorphism: Mod. U. S. Geol. Survey, vol. 47, 1904, pp. 752, 753. 
