CHEMICAL CHANGES IN WEATHERING. 37 
For comparison a composite analysis of 51 American Paleozoic shales made by 
H. N. Stokes of the United States Geological Survey « is added, the unessential ele- 
ments being also omitted. 
Composite analysis of 51 Paleozoic shales. 
Per cent. 
Silica 60.15 
Alumina 16. 45 
Ferric oxide 4. 04 
Ferrous oxide 2. 90 
Potash 3. 60 
Per cent. 
Soda l.oi 
Magnesia 2. 32 
Lime 1.41 
Water above 110° C 3. 82 
As these figures come between the extremes of the slate analyses, and as numerous 
clays and some schists do also,'' it is evident that a chemical analysis of a slate is not 
sufficiently characteristic to prove that it is not that of a shale, a clay, or a schist. 
Indeed, but for the low maxima for soda (2.17, 2.59, and 3.97 per cent) in 3 out 
af the 29 analyses, and the high minima for combined water, many granites, syen- 
ites, porphyries, diorites, and basalts could pass chemically as roofing slate. 
That there should be such chemical similarity between slate, shale, and clay results 
primarily from the fact that the slate here considered is simply metamorphosed shale 
and that shale is compressed clay. Whatever mineralogical changes metamorphism 
brought about, the same elements j .ersisted. In some mica-slates the grains of quartz, 
feldspar, zircon, and other minerals are the identical ones of the original clay sedi- 
ment. The economic bearing of the chemical analyses of slates will be brought out 
finder "Methods of testing slate," in the section oh economic geology. 
CHEMICAL CHANGES IN WEATHERING. 
The changes in substance and color which slates undergo in weathering are striking. 
rhe black slate of the Peach Bottom district (Pennsylvania and Maryland) and of 
Buckingham County, Va., passes into a bright reddish-brown clay, while the black 
slate of Lehigh and Northampton comities, Pa., passes into a yellowish ocher and 
finally into a white clay; and the black clay slate of West Virginia passes through 
similar changes. These alterations will be referred to in describing each district. 
These changes in weathering should not, however, be confounded with the relatively 
slight discoloration which slates on the roof undergo during a fraction of a century 
or even several centuries. The former changes probably required tens of centuries. 
Dr. George P. Merrill has investigated the weathering of Peach Bottom slates c 
and shown that in the passage from the black slate to the red clay the following 
losses occur: 57.57 per cent of the SiO,; 8.78 per cent of the iron oxides; all of the 
lime; 28.16 per cent of the MgO; 77.95 per cent of the K,0, and 99.64 per cent of the 
Na 2 0; while all of the A1 2 3 remains, as well as the water. A piece of Peach Bottom 
slate in the first stages of weathering, examined microscopically by the writer, shows 
the magnetite crystals and lenses passing into hematite and the andalusite crystals 
becoming limonitic from the oxidation of its ferruginous inclusions. 
In connection with some chemical analyses of slates from Lehigh County, Pa., Doc- 
tor Hillebrand made some partial analyses of a piece of black slate from a ledge near 
the Berks and Lehigh County line, l 1 , mile north-northwest of Rothrocksville, on the 
-oad from that place to Hynemansville, in which the stages of weathering were finely 
ihown. Analysis A is of the black but slightly altered part of the slate; B is of the 
greatly altered white part of the same specimen. The alteration has proceeded first 
■dong the bedding planes, which there undulate in a vertical direction and from 
r *Bull. U. S. Geol. Survev No. 168, p. 17 (B), 1900. rT , ,_ 
b For clays see H. Ries, Technology of the clay industry, Sixteenth Ann. Rept. I . S. Geol. survey, 
H. 4, pp. 523-575, 1895, pp. 554-575; also Prof. Paper No. 11. 1903. For schist, see Bull. 1 18, Analyses 0J 
locks, etc., Clarke and Hillebrand, p. 221 (L) and p. 99 (K), 1897. 
1 "Treatise on rocks, rock weathering, and soils, N. Y., 1897. 
