716 
THE GARDENING WORLD 
August 22, 1903. 
have taken an immense number oi years to form, and after 
that the sea bed again became elevated, and we come to what 
is known as the “Great Dirt Bed or Black Dirt,’ which is 
1 it. in thickness. Allowing the length of time to elapse for 
the formation of this top soil of the earth’s surface and the 
subsequent hardening by pressure to which it has been sub¬ 
jected, it must represent a considerable period of time for this 
foot of earth. The surface of the rock, we can imagine, would 
partly decay by exposure to the action of radn and the at¬ 
mosphere, while it is just possible that matter bad been 
brought from other land in the neighbourhood and added to 
by decayed vegetation. The above layer of soil is now buried 
under a depth of 17 ft., consisting of various layers of clay, 
earthy limestone, rubble, and a foot of soil, forming the present 
surface of the earth. 
Our interest chiefly centres in these dirt beds, which repre¬ 
sent the earth’s surface in ancient times, and where the fossil 
trees which grew on the surface are now embedded. Some 
of the stools occurring in these beds represent the roots, and 
of one of these roots we give an illustration. Locally they 
are known as “ birds’ nests,” because they form circular ridges, 
with a hollow in the centre, recalling the nest of a bird. 
Other fossils represent the trees standing in situ, but broken 
off near the base, and one can only surmise as to what had 
happened to cause 'the trunks to become broken off: in this 
fashion. 
The celebrated geologist, Mr. J. C. Mansel-l’leydell, gives it 
as his idea that this ancient, land surface had once more been 
on the move downwards, bringing the surface to water level, 
and converting it into. a. marsh. The effect of this upon the 
Cycads and Conifers would be to cause the trunks to decay 
rapidly at the water surface, where they came in contact with 
the atmosphere. The trees would, of course, die., and the first 
great storm of wind, or it might have been flood of water, 
had broken and washed off the tops and carried them away, 
leaving the stumps in the position where they grew, to become 
fossilised beneath the clay and limestone deposited upon them. 
The air being completely kept from them, it would preserve 
them until such time as they had become hardened and fos¬ 
silised by the infiltration of silica, in a. liquid form, later on to 
become hardened or, in other words, practically converted into 
stone. 
The Cycads which grew in these ancient, times were not 
exactly identical with those now living, but would be described 
under different generic and specific names. The earliest 
known fossils of this class were even more remote from modern 
Cycads, and were termed “ Cyeadites ”—that is, plants re¬ 
sembling Cycads. A number of species have been discovered 
from these dirt beds at Purbeck and Portland. These would 
include Yatesia gracilis, Bennettdtes portlandicus, Mantellia 
nidiformis, M. intermedia., M. microphylla, and M. pygmaea.. 
These specific names, in most cases, would give an indication 
of the general character of the fossils found, and many of them 
will be understood by gardeners. A genus of Conifers has 
also been discovered in these beds—namely, Dammarites Fit- 
toni, which would be compared to the genus Dammara, now 
found in New Zealand, the Malayan Peninsula., and other 
warm countries of the southern hemisphere; and, needless to 
say, they can only now bei grown under glass. We do not 
describe them -as identical with living species, but the fossils 
were merely the far-off ancestors of these now living. 
Unfortunately, we are not able to represent a full-sized tree, 
or even an entire trunk, owing to the destruction of the ancient 
forest of Cycads in those far-off times by the agencies above 
named. Large portions of trunks have, however, been dug 
out of these beds, and many of them have. been, set up in the 
garden at Portland Prison, and used as garden ornaments of 
a. veiy unusual kind. 
Three or more of these trunks are represented in our Supple¬ 
ment for this week. One of the trunks is stood upright in 
the centre of a. small bed, as shown a.t the foot of the Supple¬ 
ment. Various rock plants have been planted around the foot 
of this fossil trunk, and a climber ha® grown partly over it. 
Near the lamp-post seen in this same illustration is another 
trunk, laid down horizontally on a piece of stone, but a better 
view of this trunk can be obtained by reference to an illus¬ 
tration in the body of the paper, and named “ Fossil trunk of 
tree.” The top half of the Supplement shows a different set 
of fossils, including the upright trunk of a. tree standing on a 
pedestal formed by others. Several similar pieces may also 
be seen forming a. sort of rockwork, on which Aubrietias, Wall¬ 
flowers, Sedums, and other plants are grown. If it were 
possible to obtain these fossils in quantity, they would make 
interesting accessories to. rockeries in the gardens of private 
establishments, and that they would be suitable as aids to tire 
cultivation of plants we have no> doubt. The stone is very 
porous, capable of holding moisture, and cool. Some stones 
are very liable to get heated with the sun, to- the great injury 
of the plants in their vicinity, but that does not apply in the 
same way to these fossil trees,, because they consist to a great 
extent of sand and limestone. 
We might also have referred to the animal fossils occurring 
in these strata, but very few of them are shown in our illus- 
Fossil Root of Cycad. 
trations. Many of,them are stood about in this same garden, 
but as far as we can observe, only one of them occurs to the 
left hand of the top half of the Supplement, where a specimen 
of fossil Ammonite is seen, possibly Ammonites giganteus, 
which is very plentiful at Portland, and locally known as 
“ Conger Eels.” A great feature of this fossil is its immense 
size by comparison with other Ammonites. 
The other picture we have already referred to as a “ bird's 
nest,” would represent a fossil-root of a tree. Some authors 
are of opinion, however, that the so-called “ bird’s nest," would 
represent a whole tree, with its leaves, which had been 
crushed down flat in the position where it grew ; but that 
can hardly be the case, because the silting up of the old 
forest bed, if such had taken place, would have filled in the 
spaces between the leaves, and kept them intact ; or, what is 
more likely, the water and silt overlying the roots would have 
caused the death of this land plant by preventing oxygen 
from getting to the roots. On the other hand, if the material 
had been laid down sufficiently rapidly to crush down the 
foliage while the tree was still alive, every leaf would have 
been crushed and broken to. pieces. It is more feasible to 
consider that the whole mass represented the base of the 
trunk, and the mass of roots permeating the soil immediately 
around the same. The roots of living Cycads support this 
view. 
