PETEOGRAPHIC CHARACTERS. 15 
Of course these secondary or authigenous minerals were merely new combinations, 
sometimes in crystalline form, of clastic ones. Thus the hematite originated in sedi- 
mentary limonite. The andalusite (Al 2 3 ,Si0 2 ) probably originated in kaolin. 
The principal elements in roofing slates, as determined by the chemical analyses 
given in the section on the chemistry of slate | p. 34 I, are to be attributed to the min- 
eral constituents in the following way: 
Si0 2 to quartz, muscovite, biotite, feldspar, chlorite, tourmaline, andalusite, zircon, 
kaolin. 
Ti0 2 to rutile. 
A1 2 3 to muscovite, biotite, feldspar, chlorite, tourmaline, andalusite, kaolin. 
Fe 2 3 to hematite, magnetite, muscovite, biotite. 
FeO to chlorite, magnetite, carbonate, biotite. 
MnO to rhodochrosite. 
CaO to plagioclase, calcite, carbonate of lime, iron and magnesia, apatite. 
BaO to barite and some silicate. 
MgO to chlorite, biotite, carbonate. 
K.,<) to muscovite, orthoclase. 
Na 2 to muscovite, plagioclase. 
Li 2 to tourmaline. 
H.,0 to muscovite, chlorite, limonite, kaolin. 
P 2 5 to apatite. 
C0 2 to calcite, rhodochrosite, and carbonate of lime, iron, and magnesia. 
FeS 2 to pyrite. 
S0 3 to barite and gypsum. 
C to graphite; coaly matter. 
Zr 2 to zircon. 
The mineral sources of the nickel, cobalt, chromium, vanadium, and ammonia 
detected by Doctor Hillebrand in the slates of Vermont and New York have not 
been determined. 
SPOTTED SLATES. 
The spots in roofing slates have long attracted attention/' In the eastern New 
York and western Vermont slate region the purple slates often have green spots of 
circular or oval, but frequently of irregular outline. These spots sometime- occur 
only along lines of bedding and correspond to or pass into green "ribbons." In 
places, however, an entire bed of purple slate several feet thick is irregularly spotted 
throughout. The red slates are also often spotted. The spots are frequently circular 
or oval and measure from a fraction of an inch to several inches in diameter and of 
pale-green color with or without a purple border. Some of the spots, however, have 
no symmetry whatever. In order, if possible, to throw some new light on this sub- 
ject a few thin sections were prepared across small spots in directions parallel to and 
across the cleavage, and in the case of the spotted red slates chemical analyses were 
made by Doctor Hillebrand of the green center of the spot, of its purple rim, and of 
the outer red slate. 
MICROSCOPIC ANALYSES. 
An elliptical green spot, 1 by | inch, in purple Cambrian slate from the Lake 
Bomoseen Slate Company's quarry, at Cedar Point, Castleton, Vt., in a section cut 
parallel to cleavage, shows, in the green part, muscovite scales lying in all dire< tions, 
large chlorite scales, quartz fragments, carbonate rhombs, and a few irregular spher- 
ules of pyrite. In the center is some opaque, noncalcareous matter, partly sur- 
a Comparative view of the cleavage of crystals and slate rocks, by John Tyndall: Phil. Mag., vol. 12, 
Jnly, 1856. On the disposition of iron in variegated strata, by George Maw: Quart. Jour. Geol. Soc, 
vol. 24, p. 379; also On variegated Cambrian slates, by the same author, PI. XIV, figs. 29, 31, 32, Lon- 
don, 1868. Les schistes de Fumav, by Gosselet: Ann. Soc. Geol. du Nord. vol. 10, pp. 63-86, Lille., 1884; 
same author, L'Ardenne, 188S, p. 35'. Text-book of Geology, by Archibald Geikie, 4th cd., vol. !, i>. 
451 (2), 1893. Lehrbuch der Petrographie, by F. Zirkel, 2d ed., vol. 3, pp. 296-297, 1894. 
