202 T. S. Hunt—The Decay of Rocks. 
ville, near Trexlertown, Penn., long ago described by H. D- 
Rogers,” where large quantities of pyrites have been mined 
from the same openings which yield limonite, some of which I 
found still retaining the imitative forms of the adjacent pyrites, 
(from which Rogers had inferred a conversion of limonite into 
pyrites,) while the waters of the mine, like those of others in the 
‘region, were charged with sebauato acid and with iron-sul- 
phate. Another remarkable locality, where pyrites replaces 
the limonite in depth, was visible in at Seitzinger’s mine, 
near Reading, Penn., Bebe other similar cases are reported in 
the vicinity ; while at Salona in the Ni ittany valley the associa- 
tion of pyrites with limonite at this same geological horizon 
has also been notieed. 
§ 28. The association of siderite or iron-carbonate with the 
limonites of the Appalachian valley is well known east of 
the Hudson in New York and Massachusetts. This mineral is 
often manganesian and passes into nearly pure rhodocrosite. 
Examples of the association 6f siderite with limonite are also 
seen, among other localities, near Hackettstown, New Jersey, 
and near Hanover, York County, Pennsylvania. These car- 
e 
es. 
at 29. I have elsewhere considered the change in siderite under 
the action of oxydizing atmospheric waters, which proceeds like 
that in feldspathic rocks, from without inwards, and is necessa- 
rily accompanied with considerable diminution of volume, which, 
In the conversion of a siderite of specific gravity 3°6 into a 
limonjte of the same density, would equal 19°5 pee cent. 
“The evidences of this contraction may be seen in the struc- 
ture of the limonite derived from ae bicoane often forms 4 
ngy mas 
porous or spo s. In the however, of nodules oF 
blo sah cae rd conversion beginnin at the outside of 
th ex layer of compact limonite is formed, an 
theci's vr within ‘this, and still saotians till the change is com- 
ete. e void _ resulting from contraction is then found 
betewen the layers which are arran ed. like the coats of an 
formed, holding in man ny ¢ ases, more or less clay or sand, the im- 
purities of the carbonate which have been separated in the pro- 
cess of conversion into limonite. In this way are fohmet the 
* Geology of Paceavieuiad i, 265. 
