12 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



DYNAMICAL SEEIES ; TEACKS AND MAKKINGS. 



Gallery XI. At the further end of the Gallery are exhibited several 

 illustrations of forms produced by natural agencies, as a rule 

 unconnected with animal or vegetable life, and yet frequently 

 simulating fossil organisms. Some of these illustrate the 

 greater geological agents. The kind of movement that 

 Between takes place in mountain building is shown by some models 



"^e^&^T^^^ constructed by Lord Avebury and presented by him (see 

 Quart. Journ. Geological Society, lix, p 348, and Ixi, p. 345). 

 Movements of this kind naturally crumple and contort the 

 rocks, and fragments bent and folded in this way are 



Wall-case exhibited in the adjoining Wall-case. Besides crumpling, 

 6a there is a shearing action, and in some of the slates may 

 be observed trilobites greatly distorted, proving the consider- 

 able movement that the particles of rock have undergone. 

 The specimens of ''ruin marble" below are also due to 

 slight cracks and displacements of the original rock-bands, 

 a phenomenon even more clearly exemplified in a brightly 

 coloured rock from Johannesburg. The " landscape marble " 

 underneath, also probably owes its origin to subsequent 

 disturbance of the original strata, in some cases perhaps 

 combined with the action of organisms. Earth-movements 

 acting on less compact rocks, such as those containing 

 pebbles or boulders, frequently produce a striation and 

 facetting of the stones, as exemplified in some curiously 

 facetted pebbles of Carboniferous age found in the Punjab. 

 One of these is here exhibited. Beside it are boulders or 

 pebbles polished or striated or facetted, either by the action 

 of ice or by that of wind-blown sand, or even by animals 

 rubbing against them. There are also shown examples of 

 rock-weathering by other agencies, such as atmospheric 

 weathering, and borings by land-shells, sea-shells such as 

 Pholas saxicava, worms, sponges, white ants, and other 

 organisms. Among these specimens the most interesting is 

 a portion of one of the columns of the temple of Jupiter 

 Serapis at Puzzuoli, familiar to all readers of Ly ell's " Prin- 

 ciples of Geology." The marble has been perforated by 

 boring marine shells (Lithodomus), which attacked it at a 

 time when, owing to the subsidence of the land, the temple 

 had been submerged more than 20 ft. beneath the sea. The 

 floor of the temple was originally 15 ft. above the level of 

 the sea, and, since submergence, has again been raised to 

 about its original level. 



