PETROGRAPHIC CHARACTERS. 17 
Doctor Hillebrand adds this observation: 
Calculation shows that with no C0 2 there would be only enough CaO for the P.O.-,, and, further, 
that the result would be no MnO. How much FeO, if any, exists as carbonate is not indicated. If, 
after allowing for apatite, for MnC0 3 , and CaC0 3 , the remainder of the C0 2 is charged to MgO, we 
find the proportions shown in the columns below." 
M. 
Q. 
K. 
( 'a< ) • 
6. it 
4.22 
. 38 
7. 11 
4.77 
.47 
7. 93 
.! 36 
MgC0 3 ! 
MnC0 3 . 
in part FeCO :j ) . 
57 
a See Doctor Hillebrand' s chemical note, \>. 3§. 
DISCUSSION OP THE SPOTS. 
From Doctor Hillebrand's analyses it would appear that there is a decrease of the 
carbonates of lime and manganese and magnesia and of silica and rutile from the 
center of the spot outward and an increase of Fe 2 3 in the same direction. 
The main results of the microscopic and chemical analyses agree even as to the 
relative amount of pyrite. The difference in color from the green to purple and rod is 
manifestly due to the differences in the amount of hematite. Pyrite, rutile, carbon- 
ate, and tourmaline are more abundant within the spots than without them. 
Certain green fossil impressions in purple slate at Middle Granville, N. Y., may 
throw some light on the origin of these spots. In this case the effect of organic- 
matter, whether the carbonaceous matter of the lining of an annelid boring or from 
a marine alga, has been to diminish the quantity of Fe. 2 3 in the slate, and possibly 
to increase the amount of chlorite." Gosselet regards the spots as the result of the 
reduction of the hematite (Fe 2 3 ) by decaying organisms to the ferrous oxide (FeO) 
and its removal as an organic salt or as a carbonate. He observes that the green 
spots in purple tiles wear less readily than the rest of the tile, because they contain 
more quartz, and this Si0 2 he attributes to infiltration.'' 
In the spots examined from the New York and Vermont slates the marked 
decrease of Fe 2 :i is accon panied by a marked increase of carbonate of lime, iron, 
and manganese,'' and of Si( )._,, also by a slight increase, in some of the thin sections 
at least, of FeS 2 . Carbonates are also characteristic of the spots in some European 
slates. c The increase of the carbonates may be directly connected with the produc- 
tion of C0 2 by decaying organisms and the consequent decrease of the Fe 2 :! . Not 
impossibly the organism may have had a calcareous exoskeleton which was dissolved 
and then redeposited as crystalline CaC0 3 . The infiltration of Si<>, and the forma- 
tion of chalcedony may be purely secondary, and likewise the deposit of FeS 2 , or 
there may have been some precipitation of FeS 2 about the decaying organism, as 
seems to have been the case in some fossiliferous sediments. At any rate, the rim 
of intermediate compositon would be the zone in which chemical reaction was less 
effective. 
In view of all these facts and indications, the spots may be safely regarded as 
probably produced by chemical changes in the sediments consequent upon I he 
decay of organisms. 
If this be the correct view, the green ribbons, which traverse both purple and red 
slate, would correspond to small deposits of decomposing organic material that 
« See Tyndall, Maw, Gosselet, Geikie, and Zirkel, as indicated by titles given in footnote on p. 15. 
bMaw (loc. cit.) bad analyses made of dark greenish ribbons in the Welsh blue slates, and found 
that the ribbons contained 6 per <ent more SiO->, 7 per cent more ALU.;- 4 i P 1 ' 1 ' , • , '" 1 more M f.° (= 7 
times as much) , but 4 per cent less Fe -. 1 per cent less FeO, and 3± per cent less K,(> than the adjacent 
blue beds. Underthe microscope ihegreen ribbons showed more feldspar and chlorite. Heattributes 
these differences to change in sedimentation. 
cSee Zirkel, loc. cit. 
Bull. 275—06 2 
