BRITISH ASSOCIATION. 
229 
subsequent to the formation of the cleavage. In making 
observations upon the cleavage, care must be exercised not to 
confound with those phenomena the marks produced by the 
mechanical movement of one rock upon another. The shells 
in slaty rocks were generally distinctly marked by ferruginous 
lineSj caused by their decomposition, and the form of their out- 
lines was often remarkably changed. The Leptsenee in North 
Devon occasionally assumed the form of Nuculse, and the Spiri- 
fers were crumbled up, or else extremely attenuated. The Tri- 
lobites of the Llandeilo flags were found in three distinct forms 
arising from the distortion taking place in a longitudinal, trans- 
verse, or oblique direction ; this seemed to be the rusult of a 
creeping" movement of the particles of the rock along the 
planes of cleavage, the effect of which was to roll them forward, 
in a direction always uniform, over the same tract of country , 
the movement does not seem to have affected the hard shells, 
but those which were thin, as also the algse and trilobites : the 
latter are covered with little folds, parallel to the wave of motion. 
In these distorted fossils the amount of movememt might be 
estimated ; as, in the space occupied by a trilobite, it amounts to 
haK an inch. Mr. Phillips had selected these facts, bearing the 
aspect of real and general laws of structure in rocks, from a 
series of classed phenomena on the subject, because he regarded 
them as positive steps towards a mechanical theory of the series 
of changes by which the structural characters and accidents of 
position were to be determined. 
On the Permian System'^ as applied to Germany, with Col- 
lateral Observations on Synchronous Deposits in other Countries ; 
showing that the Rothe-todte-lieffende, Kupfer Schiefer, Zeclistein, 
and the lower portion of the Bunter-sandstein form one natural 
group, constituting the upper member of the Palceozoic Rocks/' by 
R. I. MuRCHisoN, Esq. 
The word Permian, when first proposed by Mr. Murchison, 
was intended to distinguish the natural group of deposits, lying 
