TACONIC QUESTION IN GEOLOGY. 227 
§ 25. The Calciferous sandrock, together with the Chazy, Birdseye and Black-River 
limestones of the Champlain division, were supposed by Rogers to be represented in the 
area just mentioned by the great masses of magnesian limestones and marbles, with inter- 
calated schists, estimated by him at from 2,500 to 5,000 feet in thickness ; while the suc- 
ceeding schists and argillites were regarded as equivalent to the Trenton, Utica and 
Loraine divisions. The large area occupied by these rocks of “the south-eastern type,” 
except in some few localities, had afforded no organic remains save the Scolithus already 
mentioned ; but the strata were supposed to have undergone a local alteration, or so-called 
metamorphism, effacing the evidences of organic life. The limestones of this series are in 
fact more or less crystalline, and often white or banded granular marbles. Moreover, 
there are intercalated both among the limestones and the quartzites of the series, peculiar 
schists sometimes containing hornblende, serpentine, tale and chlorite, besides damourite, 
pyrophyllite and other hydrous micaceous species, which have been mistaken for magne- 
sian silicates, and have caused these rocks, as a whole, to be called talcose or magnesian. 
Many of these silicates, such as hornblende, serpentine and micas, are also found in the 
limestones. 
§ 26. These Taconic limestones and quartzites include moreover large masses of iron- 
ores, sometimes a peculiar type of magnetite, more rarely of hematite ; besides beds or 
lenticular masses of pyrite and of siderite. The latter two species, in regions where the 
effects of subaérial decay are seen to considerable depths, are converted into brown-hema- 
tite ores—limonite or turgite—which are found imbedded in soft clayey and generally 
highly inclined strata, the results of the decomposition and partial solution of the lime- 
stones and their associated schists. The limonitic ores of this horizon are extensively 
mined along the outcrop of these Taconic rocks, from Vermont to Alabama; and, as has 
been shown by the concordant observations of many investigators, have been derived by 
epigenesis, in some cases from the sulphid, and in other cases from the carbonate of iron ; 
both of which, in the deeper workings, are found unaltered. Crystals of magnetite are 
sometimes disseminated through these schists, as wellas thin layers of compact hematite, 
both of which are occasionally found in the clayey beds with the limonites. The 
massive granular magnetic ores of this horizon in Pennsylvania are generally associated 
with small quantities of pyrite and chalcopyrite, and frequently yield by analysis a little 
cobalt. They are distinguished from the magnetites of the older rocks by their generally 
finely granular texture and feebler cohesion, as well as by the characteristic imbedded 
minerals. The hematite is often a very soft unctuous micaceous variety, and both 
magnetite and hematite are occasionally found in grains disseminated in the soft granular 
sandstone layers. The limonites are often manganiferous, and are sometimes accompanied 
by manganese-oxyds, which are doubtless derived from corresponding manganesian carbo- 
nates.* Associated with the limestones of this series are sometimes considerable inter- 

* The few carbonated ores from this horizon in Pennsylvania, which have been analysed, are more or less 
manganesian ; one of them yielding to McCreath 50 per cent. of manganesian carbonate. A massive fawn-colored 
carbonate, with a specific gravity of 3:25, found in layers in the so-called primordial slates of Placentia Bay in 
Newfoundland, gave me by analysis 81-6 per cent. of manganesian carbonate, and 15:4 per cent. of silica, for the 
most part soluble in a dilute alkaline solution, besides traces of ferrous, calcareous and magnesian carbonates. It 
was partially incrusted with black crystalline manganese-oxyd, evidently of epigenic origin. Amer. Jour. Science, 
1859, vol. xxviii, p. 374. 
