ON CLEAVAGE AND FOLIATION IN ROCKS. 



387 



ment uniformly in the line of the dip of the strata of the parts of symmetrical 

 fossils like trilobites, LingulcB, SpirifertB ; so that, when presented in one 

 direction, these objects were shortened, — in a direction at right angles to the 

 former they were relatively lengthened (really narrowed), and in an inter- 

 mediate direction distorted, fig. 31. And the change of figure was employed 

 as a measure of the movement on the plane of stratification, viz. :j or ^ an 

 inch in the common trilobite of Llandeilo ( Ogygia Buchii), equals xoi^i or 

 ith of the whole space. The movement does not seem, in the case of Irish 

 or North Devon rocks, to have affected the thicker and harder shells, but 

 only those which were thin, as also the Algae and Trilobites ; the latter 

 in Llandeilo flags are often covered with little folds, or even thread-like 

 striations parallel to the wave of motion, fig. 32, which, when lying right 

 across the axis of figure, may deceive an inexperienced person into the sup- 

 position of a real transverse striation. The same thing occurs in North 

 Devon, and in the south of Ireland. 



Fig. 31. 



(c.) By attending carefully 

 to the surfaces of stratification 

 and marking the phaenomena 

 on these surfaces where they 

 are modified by cleavage, an- 

 other curious and important 

 structure is indicated, which 

 appears to have escaped pub- 

 lication, though I learn with 

 pleasure that it has not been 

 unobserved by Sedgwick. 



Let S be the strike of a 

 bed, <T the strike of cleavage 

 on the surface of the bed, 

 and parallel to it (not in this 

 instance coincident with S), 

 ridges and furrows indicating 

 the internal movements of the 

 mass. 



2c2 



Fig. 32. 



