CLASTIC ROCKS 119 



be hard and solid instead of soft and loose (Figs, n and 12). 

 In many cases the relics of plants or animals, fossils, may be 

 found lying on the lamination planes or in the beds (Fig. 26). 

 In every particular the arrangement of materials seen in a sand- 

 stone or conglomerate quarry will be found to correspond with 

 that in sediment as studied in a previous chapter (compare Figs. 

 17 and 25). Thus the rock constituents are arranged with their 

 longer axes parallel to the layers, and the regularity or irregularity 

 of the layers varies with the coarseness in grain of their con- 

 stituent substances. The main points of difference will be that 

 the sandstone or conglomerate is usually consolidated or cemented 

 into a solid block, and the layers, even if regular, may not 

 infrequently be inclined to the horizontal. 



The several types of solid rocks possess characters which tend 

 to prove that they were once deposited in river beds or lake bottoms 

 or on sea floors, the likeness extending even to the types of fossil 

 animals or plants found in them, which are of kinds restricted to 

 one or other of these situations. The presence of cockles, mussels, 

 periwinkles, corals, or foraminfera will indicate that the strata 

 containing them are of salt-water origin; while fresh-water 

 mussels, pond snails, fresh-water fishes, abundant plant remains, 

 and occasional skeletons of land animals are more likely to be 

 laid down in rivers or lakes. Deposits like the Wealden beds, the 

 Purbeck rocks, most of the Oligocene rocks, the Old Red Sandstone, 

 and the strata associated with coal seams, belong to the latter class. 

 Examples of the former are naturally much more common, and 

 good types are seen in the London Clay, the Chalk, the Oolitic 

 limestones or the Lias, and the Carboniferous and Wenlock Lime- 

 stones. A study should be made of any of these or others which 

 are available, their structures noted and compared with those of 

 modern sediments, and their fossils collected in order to ascertain 

 the exact conditions of deposition. 



When it is felt that the comparison of rocks with sediments 

 is a close and accurate one the question will naturally arise as 

 to how the rocks, formed at low levels and for the most part under 

 water, have been lifted to form the skeleton of land and landscape. 

 Proof of submergence of land beneath the sea, or its elevation 

 above sea level, will now be required. 



