12 
SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
and the chlorite by pressure. Others consist of chlorite surrounded by calcite and 
that by quartz. « 
Others have a central crystal of pyrite instead of magnetite, 6 or the pyrite may 
have been changed to limonite. c Still others consist of chalcedony surrounded by 
chlorite scales,^ or of quartz surrounded by radial plates of muscovite, <° or of biotite 
surrounded by quartz (see p. 65). These lenses may, as in the Peach Bottom slates 
(see p. 87), be so minute as not to result in any visible speckling of the surface, and 
may consist of crystals of andalusite surrounded by chlorite, quartz, or muscovite, 
the orientation of the lens being different from that of the crystal. These crystals of 
andalusite may have quartz on one side and muscovite on the other. 
Finally, the discoloration once attributed by Bischof to the hydration and oxyda- 
tion of a ferrous oxide has been shown, in some slates at least, to be due to the altera- 
tion of a ferrous carbonate to limonite, and is thought to be probably due to this in 
most slates (see pp. 36, .39). 
The presence of kaolin is, of course, to be assumed in all clay -slates and also in all 
mica-slates in which the micatization of the matrix is incomplete. Its presence in 
minute quantities in those mica-slates which have little or no luster, like the Monson, 
Me., slates, and possibly the Lehigh and Northampton County slates of Pennsylvania, 
may be suspected, but not demonstrated. 
Leaving out the rarer and less significant constituents and basing his estimates on 
the chemical and microscopic analyses of the principal varieties of slate from the 
French Ardennes, Renard computes the mineral percentages as follows: 
Percentages of minerals in slates of Ardennes, France. 
Mineral. 
Purple. 
Green. 
Bluish 
black. 
Grayish 
green. 
40.09 
7. 75 
40.41 
6.23 
l . 55 
39.54 
5. si 
45.78 
2.90 
1.04 
37.75 
12. 55 
40. 58 
4.81 
37.97 
Chlorite 
17. 99 
30.97 
Hematite 
Rutile 
1.34 
Limonite 
3.09 
In round numbers this amounts to- 
Muscovite ." 38 to 40 
Chlorite ■ 6 to 18 
Quartz 31 to 45 
Hematite : 3 to 6 
Rutile 1 to H 
Renard/ calls attention to the fact that the green slates of Fumay in the Ardennes 
contain 4 per cent more Si0 2 than the purple ones and about 3£ per cent less Fe 2 3 . 
Rosenbusch,? in comparing 18 analyses of clay slates from different parts of Europe, 
calls attention to the strikingly characteristic preponderance of the MgO as compared 
with the CaO, along with a uniformly high percentage of iron oxides and of A1 2 3 , 
and also to the like preponderance of the K 2 over the Na 2 0, which he explains thus: 
Clays and clay slates constitute the finest mechanical detritus from quartz feldspar rocks; whatever 
silicates of lime they contain was removed as a soluble bicarbonate and for this reason very little 
lime-soda feldspar can occur in such a detritus. 
a Renard, op. cit., p. 248. 
bSeeHarker, on "eyes" of pyrite and other minerals in slate; Geol. Mag., Dec III, vol. 6, No. 103, 
p. 396. London, Sept., 1889. 
cLoretz, Ueber Transversalschieferung, etc., 1882, pp. 283-289. < For full titles of papers and works 
cited see Bibliography, pp. 138-145.) 
''Milliard, Sur l'examen microscopique de quelques schistes ardoisiers: Bull. soc. mineralogique de 
France, vol. 3, No. 4, 1880, p. 101. 
eZirkel, Lehrb., p. 744, Thonschiefer 
/Op. cit., vol. 2, 1883, p. 147. 
yElemente der Gesteinslehre, p. 424. 
