198 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
But by far the most interesting occurrence is that of a sort of 
mineral charcoal. Instead of the ordinary core of sand, surrounded 
by a thin layer of coal, we have in a few cases the entire trunk 
preserved as a black fibrous substance, closely resembling ordinary 
charcoal. It is, however, much more friable and crushes to a fine 
powder in the hand. Under the microscope, with reflected light, it 
is clearly seen that it is not homogeneous in composition, but, in 
addition to the carbonaceous matter, it is characterized by the 
presence of considerable amounts of a whitish mineral substance 
apparently in the form of minute spicules and fibers. In testing 
with dilute acid it was "found to effervesce freely, and an analysis 
showed the presence of over 80 per cent of calcium carbonate. The 
analysis in full was as follows : — 
Moisture, 
Calcium carbonate, 
Carbon, 
Silicious ash, 
100.25 
.11 
80.59 
8.87 
10.08 
The diameter of the trunks thus preserved was about six inches. 
The occurrence is probably analagous to that of the silicified 
lignites of the auriferous gravels of California, described by J. A. 
Phillips (Geol. mag., 10, p. 98, March, 1873), and by T. Sterry 
Hunt (Systematic mineralogy, 1891, p. 118), and probably repre¬ 
sents the first step in the replacement of the wood, which had 
already been converted into lignite, by mineral matter. But while 
in the replacement by silica, the filling of the pores is followed by a 
substitution of the silica for the carbonaceous matter itself, in the 
case in question the insolubility of this carbonaceous material, as 
compared with the calcite, presents an obstacle to the completion of 
the process. 
In 1880, Prof. W. O. Crosby and Mr. Geo. II. Barton published 
(Amer. journ. sci., ser. 3, 20, p. 419) an account of certain hollow 
molds, probably of Sigillaria, which they observed in the Carbonif¬ 
erous series of the Norfolk Basin. (See also a paper by Mr. 
J. B. Woodworth on Carboniferous fossils in the Norfolk County 
Basin in the Amer. journ. sci., ser. 3, 48, p. 145.) About thirty of 
these molds were observed and some were probed to a depth of 
twenty feet or more. They were regarded bv the discoverers as 
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