332 
ELEMENTS OF GEOLOGY. 
led the quarrymen to call them “petrified bird’snests,” which 
suggested to Brongniart the specific name of nidiformis, I 
am indebted to Mr. Carruthers for the annexed figure of one 
of these Purbeck specimens, in which the original cylindrical 
figure has been less distorted than usual by pressure. 
Many silicified trunks of coniferous trees, and the remains 
of plants allied to Zamia and Cycas^ are buried in this dirt- 
bed, and must have become fossil on the spots where they 
grew. The stumps of the trees stand erect for a height of 
from one to three feet, and even in one instance to six feet, 
with their roots attached to the soil at about the same dis¬ 
tances from one another as the trees in a modern forest. The 
carbonaceous matter is most abundant immediately around 
the stumps, and round the remains of fossil Cycadeoe. 
Besides the upright stumps above mentioned, the dirt-bed 
contains the stems of silicified trees laid prostrate. These 
are partly sunk into the black earth, and partly enveloped 
by a calcareous slate which covers the dirt-bed. The frag¬ 
ments of the prostrate trees are rarely more than three or 
four feet in length ; but by joining many of them together, 
trunks have been restored, having a length from the root to 
the branches of from 20 to 23 feet, the stems being undivided 
for 17 or 20 feet, and then forked. The diameter of these 
near the root is about one foot; but I measured one myself, 
in 1866, which was 3|^ feet in diameter, said by the quarry- 
men to be unusually large. Root-shaped cavities were ob¬ 
served by Professor Henslow to descend from the bottom of 
the dirt-bed into the subjacent fresh-water stone, which, 
though now solid, must have been in a soft and penetrable 
state when the trees grew. The thin layers of calcareous 
slate (Fig. 309) were evidently deposited tranquilly, and 
would have been horizontal but for the protrusion of the 
stumps of the trees, around the top of each of which they 
form hemispherical concretions. 
Fig. 309. 
Fresh-water calcareous slate. 
Dirt-bed and ancient forest. 
Lowest fresh-water beds of the 
Lower Purbeck. 
Portland stone, marine. 
Section in Isle of Portland, Dorset. (Buckland and De la Beche.) 
The dirt-bed is by no means confined to the island of Port¬ 
land, where it has been most carefully studied, but is seen 
