38 MINERALIZATION OF [Ch. IV. 



of fluid above. The same happens in regard to organic remains which 

 are filled with water under great pressure as they sink, otherwise they 

 would be immediately crushed to pieces and flattened. Nevertheless, ii 

 the materials of a stratum remain in a yielding state, and do not set or 

 solidify, they will be gradually squeezed down by the weight of other 

 materials successively heaped upon them, just as soft clay or loose sand 

 on which a house is built may give way. By such downward pressure 

 particles of clay, sand, and marl, may become packed into a smaller 

 space, and be made to cohere together permanently. 



Analogous effects of condensation may arise when the solid parts of 

 the earth's crust are forced in various directions by those mechanical 

 movements afterwards to be described, by which strata have been bent, 

 broken, and raised above the level of the sea. Rocks of more yielding 

 materials must often have been forced against others previously consol- 

 idated, and, thus compressed, may have acquired a new structure. A 

 recent discovery may help us to comprehend how fine sediment derived 

 from the detritus of rocks may be solidified by mere pressure. The 

 graphite or " black lead" of commerce having become very scarce, Mr. 

 Brockedon contrived a method by which the dust of the purer portions 

 of the mineral found in Borrowdale might be recomposed into a mass as 

 dense and compact as native graphite. The powder of graphite is first 

 carefully prepared and freed from air, and placed under a powerful press 

 on a strong steel die, with air-tight fittings. It is then struck several 

 blows, each of a power of 1000 tons ; after which operation the powdei 

 is so perfectly solidified that it can be cut for pencils, and exhibits when 

 broken the same texture as native graphite. 



But the action of heat at various depths in the earth is probably the 

 most powerful of all causes in hardening sedimentary strata. To this 

 subject I shall refer again when treating of the metamorphic rocks, and 

 of the slaty and jointed structure. 



Mineralization of organic remains. — The changes which fossil organic 

 bodies have undergone since they were first imbedded in rocks, throw 

 much light on the consolidation of strata. Fossil shells in some modern 

 deposits have been scarcely altered in the course of centuries, having 

 simply lost a part of their animal matter. But in other cases the shell 

 has disappeared, and left an impression only of its exterior, or a cast of 

 its interior form, or thirdly, a cast of the shell itself, the original matter 

 of which has been removed. These different forms of fossilization may 

 easily be understood if we examine the mud recently thrown out from a 

 pond or canal in which there are shells. If the mud be argillaceous, it 

 acquires consistency on drying, and on breaking open a portion of it we 

 find that each shell has left impressions of its external form. If we then 

 remove the shell itself, we find within a solid nucleus of clay, having the 

 form of the interior of the shell. This form is often very different from 

 that of the outer shell. Thus a cast such as a, fig. 58, commonly called 

 a fossil screw, would never be suspected by an inexperienced conchologist 

 to be the internal shape of the fossil univalve, b, fig. 58. Nor should 



