THE STRUCTURE OF ROCK MASSES. 123 
the cells being in this case flattened and drawn out, so that the 
longer axis lies in the line of the cleavage. 
In the same manner the yellow concretions, which look like so 
many blotches in the Welsh slates, when found in uncleaved 
slate rock, are seen to be nearly spherical or ellipsoidal con- 
cretions occurring in the lines of stratification, and which, when 
they are longer in one direction than another, invariably' have 
their major axis lying in the plane of bedding. In the cleaved 
slate, however, they are found to be considerably changed, as 
seen in fig. 5 a and 6, PI. LVII., where they are represented as 
flattened out and compressed sidings ; the major axis of these 
now always follows the direction of the lines of cleavage. 
The peculiar effect of cleavage in altering the form of fossils, 
as has previously been noticed, has proved a puzzle to the 
paleontologists, who, deceived by appearances, have frequently 
given different names to distorted specimens of the same fossil. 
This was the case with the two names Sjpirifer disjunctus and 
JSjo. giganteus, the latter being subsequently shown only to be 
the former greatly distorted and expanded by the effects of the 
cleavage in. the slates at Tintagel in Cornwall. In like manner, 
the shell Euomphalus jpentangulatus , from Little Island, Cork, 
represented by the figures 12 a, 6, and c, PI. LVII. (from 
Haughton), had frequently (owing to the distortion and conse- 
quent change of form from their normal appearance, fig. 12 b) 
been described as Ellipsolithes , and considered as quite distinct 
from Euomphalus ; and several other quite similar mistakes 
could be mentioned. 
Another example of how greatly the appearance of the same 
fossils, when occurring in cleaved rocks, may differ from one 
another will be seen in the case of the trilobite Asaphus 
Homfrayi (Salter), shown in fig. 9 a b, PI. LVII. 
A comparison of the normal and distorted fossils proves, with- 
out doubt, that they must have not only been subjected to a 
compressing action, but also that this has at the same time been 
combined with a sort of sliding movement, which has caused 
them to become drawn out or elongated along the planes of 
cleavage in one direction much more than the other. As an 
experimental illustration of how such an effect can be produced, 
the changes which a coin, in this case a farthing, w T ill expe- 
rience when passed through a rolling-mill, are shown in fig. 
6 a , b, c, PL LVII., and which will not require any explana- 
tion. The action of the rolls upon a substance inserted between 
them is a combination of direct pressure with forward move- 
ment, and consequently not only causes the raised impression 
on the coin to become flattened out, but also at the same time 
elongates its one axis in the direction of the forward movement, 
i.e. that in which it had been inserted between the rolls, whilst 
