Prof. Sedgwick on the Structure of large Mineral Masses. 463 



mechanical, stratified rocks by causes acting under a comparatively low tem- 

 perature, and often during indefinite periods of time. Very few of these 

 changes can be imitated in a laboratory, because it is impossible to imitate 

 the conditions under which they have been brought about. They admit not, 

 therefore, of synthetic proof; but they are unc|uestionably subordinate to che- 

 mical and mechanical laws, which we can study experimentally, and estabhsh 

 on appropriate evidence. By assuming the existence of these laws, and by 

 studying the conditions under which they have acted (not as matters of ex- 

 periment but of observation), we may gradually ascend towards an explanation 

 of some of the perplexing phenomena presented by sedimentary rocks. 



That these rocks are greatly changed since their first origin is too obvious 

 to require any formal proof. Take, for example, a mass of Hertfordshire 

 pudding-stone. There can be no doubt that it once formed a portion of a 

 shingle bank of rolled chalk flints and finely comminuted sand. The mate- 

 rials are now so closely agglutinated by siliceous cement, that a fracture passes 

 indifferently through the sandstone and the imbedded flint pebbles. Again, 

 rocks of this kind are sometimes divided into prismatic masses by cross joints ; 

 and these joints pass without any deviation through the imbedded flints, so as 

 to produce a series of smooth surfaces. I merely mention this as an example 

 of a great change produced naturally (and uncjueslionably without any very 

 high temperature) upon a coarse, mechanical deposit. 



Again, every bed of secondary limestone, containing organic remains, gives 

 us an example of somewhat similar changes. We find a mass, hard and solid, 

 which was once comparatively soft and incoherent; we find crystalline struc- 

 ture, and sometimes even regular cleavages, where such an arrangement of the 

 parts could not have originally existed ; and, what is more, we find parts ori- 

 ginally solid replaced by other solids — a trunk of a tree, for example, replaced 

 by nearly pure silica — a shell by a similar substance, or more commonly by 

 pure carbonate of lime with a regular crystalline structure. The solidification 

 of calcareous beds is easily explained, and the process may be imitated. The 

 replacement of one solid by another requires conditions it is difficult to imitate : 

 but the fact shows that an enormous extent of chemical action, enough indeed 

 greatly to modify the whole aspect of external nature, may take place among 

 the solid parts of bodies without an approach to direct chemical solution. 



When we consider even such simple examples as those just quoted, they 

 give us an impressive proof of the powers of nature in producing mineral 

 changes ; and when we bear in mind that some of our secondary strata may 

 have rested thousands of years under the pressure of a thousand atmospheres, 

 we need not wonder at the extent to which such changes have been carried. 



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