* =i he > 
TRANSACTIONS OF THE SECTIONS. 61 
vided by the planes of cleavage. Several attempts had been made to imitate cleavage 
structure. Mr. Fox of Cornwall, had caused electric currents to pass through moist- 
ened clay, and had thus produced fissures parallel to the bounding surfaces of the 
mass, and this illustration Mr. Phillips considered very important, but incomplete. 
The cleavage planes of the slate-rocks of Wales were, he stated, always parallel to 
the main direction of the great anticlinal axes, but were not affected by the small un- 
dulations and contortions of those lines. In North Wales they maintained the same 
direction for fifty miles, not varying more than two or three degrees, which might be 
owing to local causes, such as the movement of masses by gravity at a period sub- 
sequent to the formation of the cleavage. In making observations upon the cleavage 
of rocks, care must be exercised not to confound with these phenomena the marks 
produced by the mechanical movement of one mass of rock upon another. The layers of 
shells in slaty rocks were generally distinctly marked by ferruginous lines, caused by 
their decomposition, and the form of their outlines was often remarkably changed. ‘The 
Leptenz in North Devon occasionally assumed the form of Nuculz, and the Spirifers 
were crumpled up, or else extremely attenuated. The Trilobites of the Llandeilo flags 
were found in three distinct forms, arising from the distortion taking place in a longi- 
tudinal, transverse, or oblique direction ; this seemed to be the result 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 coun- 
try: the movement does not seem to have affected the hard shells, but only those 
which were thin, as also the Alge and Trilobites : the latter are covered with litile folds, 
parallel to the wave of motion. In these distorted fossils the amount of movement 
might be estimated ; as, in the space occupied by a trilobite, it amounts toa quarter 
or even half 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. 
Notice of the Ordnance Geological Museum. By Prof. J. Putwuirs, F.R.S. 
The Ordnance Geological Survey, now under the direction of Sir Henry De la Beche, 
has for its object two purposes, both bearing on geological science. The first of these 
is to procure an accurate delineation of the boundaries of all the strata; not laid down 
conjecturally, but from actual observation and exact measurement. For this purpose 
there is attached to the Trigonometrical Survey a staff of active geologists, who walk 
over the boundary lines and draw sections, in which every part is measured and laid 
down on a scale true to nature. The practice of making the vertical scale of sections 
greater than the horizontal had led to much mischief and many mistakes, especially in 
mining operations, and in working for coal. The second object was to form a collection 
of the fossils of every stratum in every favourable locality, with the view to ascertain 
the law of geographical distribution of species, and their geological distribution in 
successive strata. It was usual for geologists to collect only fine and beautiful spe- 
cimens ; but this method would never enable them to trace the exact areas of distri- 
bution of organic life in former epochs. In connexion with this system of collecting 
organic remains, a method of drawing them had been proposed: each species, as 
soon as sufficient information could be procured, was to be drawn by good artists, 
With magnified views of minute structure; and these were to be engraved on steel, a 
separate plate being given to each species, and published at so low a price as to place 
them within the reach of all persons interested in the science. Mr. Phillips invited 
collectors of fossils to give their aid in forwarding this work, by lending specimens of 
Tare or new fossils, and announced that in such cases electrotype impressions would 
Gf approved) be taken of them, so as to supply museums and collectors with correct 
representations of unique or valuable fossils. In conclusion, Mr. Phillips stated that 
the Ordnance Museum would be eventually opened to the public as freely as any in- 
stitution of the same nature. 
