1815,] On. Septaria, 409, 
space of two or three weeks the detached portions acquired in many 
instances almost a flinty hardness. These are broke into small 
ieces of about 2 lb. weight, placed in a kiln similar to a lime- 
kiln, and exposed nearly to the same degree of heat: when re- 
moved from the kiln, they are reduced to a fine powder in a grind- 
ing mill, and then constitute the same cementing material as what 
is known under the name of Larkin Cement. ; 
The cliff, when examined in situ, has all the appearance of an. 
uniform argillaceous mass, except in some parts, separated by a 
whitish thin horizontal layer, which consists principally of carbonate 
of lime, Most of the portions at the period of being detached 
have an homogeneous appearance. During the period of desicca- 
tion a very curious change takes place. ‘The exterior part hardens 
first, to a certain extent ; and as this change is advancing towards 
the central portion, a fissure is produced, and the carbonate of 
lime, which retains its soft state much longer than the argillaceous 
portion, is mechanically separated from the clay, and pressed into 
this divided part. In its first stage I have remarked it to be near 
half an inch below the surface of the detached portion. In this 
state it is soft, but not fluid. After two or three days the calcareous 
matter becomes leyel with the surface, and in many instances pro- 
jecting above. As the carbonate of lime hardens, a species of erys- 
tallization takes place, from which cause there is an additional pro- 
trusion of the substance, During these processes there are trans- 
verse fissures in different directions, considerably smaller than the 
first separation. These smaller clefts are filled in a similar manner, 
When perfectly hardened, and cut through, a complete septarium 
is observed to have been formed, It appears that the carbonate of 
lime is mechanically mixed with the other portions; and when it 
exists beyond a certain proportion, from remaining softer longer 
than the other parts, becomes mechanically pressed in that direc- 
tion where it meets with the least resistance, viz. the central part: 
when the proportion of carbonate of lime is small, it remains inter- 
mixed with the clay. From this circumstance we observe many of 
the hardened portions without any calcareous septa. 
It appears probable that metallic veins may be formed by a 
similar process; in the first instance an apparently homogeneous 
soft mass; and that during the subsequent periods of consolidation, 
the metallic mass, undergoing this change more slowly, may be 
similarly determined in any fissure formed by the desiccation of the 
other materials. If different metallic substances should require 
different periods for consolidation, we may form some idea of the 
formation of cross courses. 
I moulded into an oblong form some of the detached portions of 
the cliff; the first-formed fissure was longitudinal ; such was to be 
expected, as the resistance in that direction would be the least. 
In Cornwall we observe the principal metallic veins are copper, 
and these are generally in the direction E. aod W If we consider 
Cornwall as a large oblong mass, the longitudinal diameter will be 
