

DISTURBED STRATIFICATION. 75 



but principally marine, sandy, and argillaceous, and with some cal- 

 careous deposits, abounding in shells and other organic exuviae, closely 

 analogous to existing species. 



Secondary or Mesozoic Group of Strata are principally of marine 

 origin, with rare and local estuary deposits ; consisting of repeated 

 alternations of limestone, flint, sandstone, sand, clay, iron ore, coals, 

 salt, &c., with organic remains, generally very distinct from existing 

 forms of animals and plants. 



Primary or Palceozoic and Hypozoic Strata. The Paleozoic rocks 

 contain organic remains,- mostly of marine tribes, and the species are 

 all extinct. 



Disturbed Stratification* 



Strata originally Level. All strata, says Cuvier, in his admirable 

 " Discourse on the Revolutions of the Globe," must necessarily have been 

 formed horizontally; and this opinion, founded upon the admission 

 that rocks composed of regular layers, containing rounded pebbles and 

 organic remains of water-animals, can only have been formed under 

 water, is supported by observation. For not only do we see at the 

 present day the deposits from water arranged in planes nearly or exactly 

 horizontal, but we also find the ancient strata of the earth, where un- 

 disturbed by convulsions, very nearly level. In consequence of these 

 disturbances the strata are seldom found to be perfectly horizontal, but 

 are often inclined at high angles, and in a few instances stand directly 

 vertical. Their planes are generally continuous over large spaces, but 

 they are sometimes broken and dislocated by faults or dykes. It is 

 now generally admitted that the usual horizontal disposition of the 

 strata is derived from the action of the supernatant waters which ac- 

 cumulated them; and that the irregular declinations and fractures 

 which we sometimes behold are the effects of subterranean convulsions 

 or changes chiefly occasioned by internal contraction. All strata which 

 were deposited in continuous sequence so as to rest evenly upon each 

 other are said to be conformable, and the succession is termed con- 

 formity. These show no evidence of changes affecting the general 

 directions of the coast lines, or form of the sea-bed at the time of their 

 deposition. There is no evidence of destruction of the beds previously 

 formed, and the interval of time between the beds was probably 

 short 



Subsequently Disturbed. Earthy matter deposited from water by 

 tranquil subsidence, as clay and limestone, or accumulated during 

 periods of moderate agitation, as sand and sandstone, must in general 

 be arranged into layers or strata, proportioned to the intervals of 

 deposition ; and these layers, in consequence of the fluctuation of the 

 water and the influence of gravitation, will especially tend to be hori- 

 zontal Nevertheless they must, in a considerable degree, accommodate 

 themselves to the surface on which they are deposited. If the bottom 

 be level, so will be the deposit ; if sloping, the deposit will be inclined ; 

 but if there be a perpendicular subaqueous cliff, no deposit can fall 



