370 



DYNAMICAL GEOLOGY. 



and become compressed, in the direction of the pressure, and extended at 

 right angles to it; and. other earthy beds have suffered more or less in a like 

 way. But strata of quartz sands, not firmly cemented, have accommodated 

 themselves to the pressure in part by rearrangements of the grains; and those 

 of limestone, and hard quartzyte, brittle rocks, mostly by fracturing, displace- 

 ment, and recementation. 



The distortions of fossils vary according to the relation in position be- 

 tween the planes of bedding or cleavage of the rock, and the axial plane at 

 right angles, or nearly so, to the direction of pressure. The inequalities in 

 the pressure and in the varying resistances to motion were a cause of a warp- 

 ing of the beds on a large scale, which had its effects. Hence stretchings, 

 slippings, and contractions of fossils are common in such beds. 



Some examples ai-e shown in the following figures from a paper by D. Sharpe (1847, 

 Q. J. G. Soc), illustrating cases observed by him in a slate rock in Wales. They repre- 

 sent two species of shells, the Spi- 

 344. rifer disjunctus (Nos. 1 to 4) and 



the Spirifer giganteus (Nos. 5 to 

 8). No. 1 is the natural form of 

 8. disjunctus ; the others are dis- 

 torted. The lines zz show the 

 lines of cleavage in the slate: 2 

 lay in the rock inclined 60° to the 

 planes of cleavage, and is short- 

 ened one half ; 3 lay obliquely at 

 an angle of 10° or 15°, and short- 

 ened above the middle and length- 

 ened below it ; 4 is a cast, the 

 upper part pressed beneath that 

 shown, while the lower is much 

 drawn out ; 5 is like 3, the angle with the cleavage-plane being less than 5°, and the lower 

 part has lost its plications by the pressure and extension ; 6 has a similar angle to the 

 cleavage-plane, but a different position ; 7 intersects the cleavage-plane at only 1°, and its 

 lower part is very much elongated. Compression, a sliding of the rock at the cleavage- 

 planes, and more especially a spreading of the rock itself under the pressure, are the 

 causes which have produced these distortions. All fossils are liable to become similarly 

 misshapen under the same conditions. 



3. Foliation, slaty structure. — Roofing slates exemplify cleavage-struc- 

 ture, or foliation. They are most common on the outskirts of regions of 

 disturbance. Slaty cleavage often graduates into the foliated structure of 

 hydromica and mica schists. The fact that slaty structure is not coincident 

 with the bedding-planes was explained by Sedgwick in 1835, from observa- 

 tions in Wales. Sorby first pointed out (1849) that the structure was due to 

 the forcing of all flattened and linear particles into parallel planes, approxi- 

 mately perpendicular to the pressure ; and that all air-cavities and particles 

 of moisture are flattened likewise. He sustained his conclusions by micro- 

 scopic examinations, and by subjecting to pressure clay and scales of oxide 

 of iron. Tyndall rendered beeswax, clay, and other substances, laminated 



