410 On Septaria. (Dec. 
in that direction, while the cross courses are generally filled with 
some other material. We may hence conceive why the metal- 
liferous lime-stone of Derbyshire may be separated by amygdaloidal 
strata called toad-stone. ‘The metallic vein in the upper stratum of 
mountain lime-stone is cut off, and again appears in the calcareous 
bed below the toad-stone stratum. Upon the foregoing principles 
we may explain the formation of veins without having recourse to 
the Huttonian or the Wernerian theories ; and Professor Playfair 
has adduced the septarium as a strong illustration of its arrange- 
ments depending upon its previous fluid state by the agency of heat. 
The carbonate of iron nearly constitutes one-third of the argil- 
laceous mass; and its cementing powers depend upon the large 
proportion of this metallic matter. With the portions of the cliff 
are found great quantities of pyrites in thin flattened portions, 
which have all the appearance of having been previously in a soft- 
ened state, as they bear the marks of impression from the surround- 
ing materials: and it is not uncommon to find portions of wood 
strongly indented in them, and in that condition as to render it 
impossible that they could ever have been exposed to the agency of 
heat. In such abundance this sulphuret of iron exists, as to largely 
contribute to the supply of a copperas work in the vicinity of Har- 
wich. Although the quantum of pyrites is so considerable, yet no 
traces of the sulphuret of iron are discoverable in the argillaceous 
mass. The formation of these pyritical masses favour the opinion I 
have ventured to offer relative to the formation of veins. If we 
regard all the substances which constitute the cliff to have been ina 
very softened state, and to have been formed of a general admixture 
of alumina, silex, carbonates of lime and of iron, and of the sul- 
phuret of iron; in the first stage of desiccation the sulphuret of 
iron appears to be the first portion thus mechanically separated. As 
the complete consolidation cannot take place in situ, on account of 
the retentive power of the large proportion of alumina for water, as 
soon as portions of the cliff are detached and exposed to the air, the 
hardness above noticed takes place, with the changes already par- 
ticularized. 
In the vicinity of Bath we have frequent opportunities of ob- 
serving in our rocks vegetable and animal remains in such conditions 
as to favour the supposition that the surrounding mass has been in a 
similar softened state. Lately a stratum of blue lyas at Twyerton 
has been worked for the purposes of repairing the roads, In this 
rock some ammonites of nearly 12 inches in diameter have been 
discovered, most curiously compressed out of their usual spiral 
direction, without any destruction of the general characters of the 
cast of the shell of this supposed sea snail. In the fissures of this 
lyas bed there are numerous beautiful octohedral crystals of pyrites, 
and in the same clefts are found large portions of wood, which have 
all the appearance of having been softened and flattened, the lig- 
neous character in many parts well preserved, while the greater 
portions are converted into beautiful jet, I have one of these pieces, 
