44 
POPULAR SCIENCE REVIEW. 
The quarry and the cutting tell a different tale. In a cutting 
of the lias, such as are seen in Leicestershire, or at "^Vhitby, or 
at Lyme Regis, or elsewhere, the greater part is clay, but there 
are bands of different material. The original clay mud was 
mixed, no doubt, with carbonate of lime mud, but now belts 
of nearly pure carbonate of lime are seen at intervals. These are 
generally collected round certain remains of animals, generally 
fishes or marine reptiles. The mud is not altered in other 
ways, but parts once mixed with the rest have settled, as it 
were, into particular strata. And something of this kind is 
seen in every cutting, not only of clay, but of limestone and 
sandstone. There is much more order and system than in a 
recent accumulation, and the order and system are chemical 
rather than mechanical. 
The amount of change observable in all these cases has cer- 
tainly been brought about after the original deposit of the strata, 
without any m.echanical or external action, and without much 
heat having been brought to bear upon them. It is a change, 
however, that has often entirely re-arranged the particles of the 
mass, and while sometimes so small as not to have altered the 
most dehcate details of a fragile shell, has entirely re-modelled 
the composition and arrangements of thousands of tons of rock 
in a comparatively small area. 
Coincident with these remarkable changes in the accumulated 
masses that make up a comparatively simple rock must be 
named the formation of cracks and fissures, no doubt the 
result of the contraction of the whole when water was parted 
with in the act of drying. And that this water may be parted 
with to some extent, even while the whole is covered with water, 
there is no doubt. Solidification is itself a definite change, 
and a partial metamorphosis. 
The filling up of the cracks in rocks is a process so mani- 
festly subsequent to the solidification of the mass, that the state- 
ment of the fact is sufiicient. The fissures are often closed 
with the same mineral as the rock, but generally in a crystal- 
line state, the crystals shooting out from the sides towards the 
middle of the cleft, and there meeting so as to leave no space. 
More frequently there are spaces left, and these are occupied 
by other crystals, identical in composition with one or more 
of the accidental contents, or impurities of the rock. Thus in 
limestone the cracks are filled for the most 23art with calc spar, 
but there are often beautiful crystals of quartz shot so as to 
fill up spaces. Iron pyrites, fluor spar, ores of lead and zinc, 
and other foreign substances, and sometimes native gold, are 
occasionally entangled among the calc spar in a singular 
manner. 
The gradual transition of chalk into white limestone, and of 
