18 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
effected similar changes in the Fe 2 3 of the argillaceous sediments. Where a bed of 
quartzite forms the center of such a ribbon quartzose sedimentation must have taken 
place also, and possibly may have been the very condition which proved favorable 
to marine life. 
IGNEOUS SLATES. 
Most remarkable are the green slates from the English Lake district (Buttermere, 
Tilberthwaite, etc.), which consist of volcanic ash and which have long been known 
in England as excellent roofing material. These have recently been chemically 
and microscopically analyzed and described. a These slates are found to consist 
chiefly of chlorite, calcite, quartz (mostly secondary), and muscovite, but contafn 
also andesitic lapilli, feldspar, garnets, sphene, and anatase. Slate needles and tour- 
maline are conspicuously absent. The chemical analyses show the following impor- 
tant constituents: 
Si0 2 50. 16-54. 02 
A1 2 3 11 . 94-17. 85 
CaO 3. 67- 6. 46 
FeO "..... 5. 97- 7. 06 
C0 2 2. 45- 5. 41 
C0 2 if calculated to CaCOg would give from 5.56 to 12.29 per cent of CaC0 3 . Spe- 
cific gravity ranges from 2.775 to 2.788. The percentage of Si0 2 is low, and that of 
FeO is near that of the "unfading green" slate of Vermont (p. 107). 
Still more remarkable are the slates first described by Mr. E. C. Eckel in 1903, h 
which seem to have been formed directly from igneous rock by shearing. They 
show from 3 to 9 per cent less Si( > 2 than the English ash slates and more than double 
the per cent of MgO of the European and American roofing slates of aqueous-sedi- 
mentary origin, analyses of which are given under the heading Chemistry of slate, 
(p. 34). These slates are fully described by Mr. Eckel on page 57. 
The state of science on the remaining structural, geological, and chemical features 
of slate will be given under the three headings which follow. 
STRUCTURE OF SLATE.' 
BEDDING. 
Ordinary planes of bedding may be defined as those which are approximately 
parallel to the surface of the water in which the sediment is formed. If the sedi- 
ment changes in character, then a horizontal bed of different material results. If, 
deposition being interrupted, annelids creep over the bottom or algae decompose 
there, and the same kind of sedimentation be afterwards resumed, then two hori- 
zontal beds of the same material will result, separated by a plane covered with trails 
and impressions. Some bedding planes are due to the changing size or arrangement 
of the particles, some may be the result of contraction in drying, others are the 
effect of vertical compression. Whatever explanation may be offered for bedding; 
the bed is the starting point in a slate quarry, for the direction of the bed indicates 
(cleavage, etc., being equal) where the same quality of slate is likely to recur. 
In those places where folding has brought the bedding plane to the angle of inclina- 
tion required by the cleavage, the unaltered sedimentary particles of the slate still 
retain their parallelism to the bedding, i. e., cleavage and bedding are parallel, but 
in all other places cleavage is necessarily more or less transverse to bedding. This, 
a Hutchins, Maynard, The ash slates of the Lake District: Geol. Mag. London, 1892. Reade, T. Mel- 
lard, and Holland, Philip, The green slates of the Lake District, with a theory of slate structure and 
slaty cleavage: Proc. Liverpool Geol. Soc. (1900-1901), 1901. 
bBull. U. S. Geol. Snrvev No. 225, p. 4l9; also "On a California roofing slate of igneous origin," in 
Jour. Geol., vol. 12, p. 15, Jan. -Feb., L904. 
'■This chapter in its original form appeared in part 3 of Nineteenth Ann. Kept. U. S. Geol. Survey, 
in 1899, but its matter is here revised and brought up to date. 
