Farming of Dorsetshire. 
395 
but the fiesh-water deposits prevail. These beds abound with 
organic remains. Turtles, Lopidotii (a genus of extinct fresh- 
water fish) have long been known among the former inhabitants 
of these ancient seas ; but the discovery of insects was reserved 
for a much later period. In the last four years remains of cole- 
opterous and hymcnopterous insects have been found in great 
abundance. An echinoderus, the only one of that genus hitherto 
recognised as belonging to the Purbecks, was found by Pi'ofessor 
E. Forbes about the same time. He has named it the Hemici- 
daris Purbeckiensis : its locality is a narrow gore on the summit 
of the cinder-bed, composed chiefly of Ostrea distorta. The 
stone from these beds is shipped off to all parts of the country, 
and is used for building, both for ornament and use. The 
well known Purbeck marble, with which the lovers of ecclesias- 
tical architecture are familiar as forming the elegant shafts and 
columns of the Early English period, may be favourably exa- 
mined in its original position near the Preventive Steps leading 
to the shore of Durles'on Bay. Amongst the debris of locks at 
Gadcliff, where the junction with the oolite occurs, lies a noble 
specimen of a fossil tree, of the natural order of Cycadea?, which 
has fallen from the overhanging cliff, and is evidently a member 
of an extensive forest, of which others may be seen at Portland 
and Lulworth. Tlie Cycadese require a more tropical climate 
than is to be met with in these latitudes. The tree in question 
is encased in a bed of limestone: the pith is discernible, sur- 
rounded with fascia? or bundles of fibre, which formed its trunk ; 
the bark too is well defined. In the clays are found bones and 
vertebrae of the Iguanodon, and of more than one laige species of 
Plesiosaurus, with those of large and small crocodiles. The 
clays also contain lignite. The cliffs are productive of much 
mineral wealth, the sliale of the Kimmeridge clay producing a 
variety of valuable products. The base of these shales is alu- 
mina ; they contain a large proportion of organic matter, are 
combustible, and when ignited burn with a bright flame ; but, 
owing to the large quantity of earthy matter contained in them, 
they cannot be employed as fuel. Wlien heated to redness in 
the open air the organic matter is slowly burned away with a 
smoky flame ; a bulky ash remains, consisting principally of 
alumina, with a small quantity of oxide of iron. The quantity 
of alumina is so large, that some of the shale has been used as a 
source of alum, and a former possessor of the property erected 
works for this purpose. If, instead of being heated to redness in 
the open air, tlie sliale be heated in a close vessel, such as a gas- 
retort, the bituminous matter will be decomposed, and its ele- 
ments resolved into a variety of gaseous or liquid compounds, 
which may be distilled off and collected. The liquid portion 
