It 



STRATIFICATION. 



STUATIKICATION. 



to falling in wmter and drifting by wind or water, the specific gravity 

 ml the magnitude of the masse* require to be taken into account. 



For our prevent purpoee it ii sufficient to observe that in a mixture 

 of pebbles and nod, subject to drifting, there is an angle of ascent 

 which limit* the movement of the pebbles, and yet allows the sands 

 to pan on. Hence, on the sea-const, the almost stationary pebbly 

 beach is margined on the landward side by parallel ridges of moving 

 mad-bill*, and thus a separation ia effected of the bases of conglome- 

 rate* and the bases of sandstones. 



If now, instead of comminuted materials falling through air on the 

 land or drifted by wind over it* surface, we imagine such materials 

 falling on and sinking in lakes perfectly calm, or the sea always in 

 motion, other effect* will follow, and otlier forms of stratification will 

 result In a calm lake the sediment mny be supposed to fall vertically 

 downwards, and, except the bed of the lake be very irregular, to settle 

 in layers or strata of considerable uniformity. The least lateral 

 movement, by mixing the suspended matter through the water, would 

 tend to widen the de|>oeit, and to fill up the in> qualities of the lake- 

 bed, to that by many repetitions a very irregular subaqueous surface 

 would be equalised aud levelled. This effect is perhaps actually wit- 

 nessed in the sediment called traps which lies in the valley of Brobl, 

 near Andernach ; this is apparently derived from volcanic dust strati- 

 fied in the water which received the shower of ashes. (Pig. 6.) Besides 



Arrangement of ashes falling In a lake* 



showers of aches, lakes and the sea receive comminuted mineral sub- 

 stance* from shore* wasted by tides and storms, and from rivers which 

 transport detritus from the interior of the country. In a very calm 

 lake the sediment from the influx of rivers subsides under the influence 

 of gravitation downward* and communicated motion forwards, so that 

 kind of conical stratification in a delta continually advancing with 

 a level top into the lake is the result In this delta the arrangement 

 of the strata i* probably such a* to exhibit successive layers dipping 

 forward into the lake, but there may also be some more horizontal 

 line* depending on the separation of the coarser and finer sediment, 

 owing to their unequal falling velocity in water. Moreover the coarser 

 sediment will fall near the point where the stream enter*, and the 

 finer will bo longer suspended in water, be transported farther, and 

 re*t on more horizontal surfaces, and thus a new complication of the 

 strata will arise, a* represented in fig. 7. 



Fig. 7. 



Deposition of coarse gravel and sand (j) ind fine clay {) in a lake by a 

 river (r). 



The i arthy material* poured into the tea by rivers follow the same 

 general law* of distribution, but aro subject to the additional effects 

 of tide* and storms. The effect* of these agencies are exhibited in 

 gnat distinctness on coast* which confront violent winds and the full 

 oceanic flow, and which are margined by pebbles and sands. The 

 and* with ripple-marked surface* gently inclined extend to some 

 distance below low-water, the pebble* spread upward* in more steeply 

 inclined bank* thrown up and left by the breakers in heaps, long 

 parallel ridges, and terrace*. A contemporaneous surface of deposition 

 iu fuoh ca*e may be a* represented in Jig. 8, where clay or sand below 



Fig. 8. 



Oravtl beaches (/), sand beds (>), and nnc clar (e) la the sea, under the 

 of Uutidrs. 



low-water, and pebble* between low- and high-water, appear in a con- 

 Unuou* layer. Such a surface differs from that found under a calm 

 lake by the concave slope of iu pebbly lamina; toward the shore, while 

 the lake deposit ha* convex slope* in the same part. 



Bach appearance*, at least partially, may perhaps be recognised 

 among the strata accompanying coal, and in other group* of rock* 

 Where the unequal sin of the granular material* and ancient exposure 

 of theee to littoral agitation supply the necessary conditions. 



Near the shore various cause* locally in operation ore known to 

 produce on the sea-bed that oblique and variously-directed lamination 



which ha* been already noticed a* an effect of wind (.tig. 4), and may 

 be seen as an ordinary occurrence in the sections of diluvium, mul in 

 the bonk* of rapid rivers which change irregularly their channel 

 (Lyell, ' Principle* of Geology.') Among the stratified rocks this 

 appearance ia common. It occurs in Sussex among wealden strata 

 supposed to be of fluviatile origin ; at Nottingham in new red-sand- 

 stone, which contains no internal evidence of the nature of the water 

 in which it waa arranged ; in millstone grit of Yorkshire, and in the 

 oolite of Weaton near Malton, and Old Down near Hath, which are all 

 deposits from the sea. It i* perhaps impracticable to determine by 

 mere inspection of the oblique lamination alluded to, the exact order 

 of phenomena which occasioned it Tidal agitation, fluviatile violence, 

 the force of temporary inundations, sea-currents, each and all of then 

 may have been concerned in the phenomena of this nature, which are 

 so frequent in pebbly sandstone*, and in general we may be justified 

 iu believing that such appearances may be safely ascribed in meet 

 coses to irregular violence of shallow water ; while in regard to the 

 more regular and extended parallel lamination of ordinary micaceous 

 sandstones, and still more the fine uniform deposits of clay and argil- 

 laceous limestone, must be referred to the wider and gentler influence, 

 and longer suspension due to calmer, that is to say, generally deeper 

 water, farther from shore. 



The changes to which the sedimentary deposits, which were the 

 origin of stratified rocks, have been subject since their aggregation, 

 are so considerable, as to gain for them, in extreme case*, the 

 distinctive title of Metamorphic Rocks. 



Consolidation of mass is the first of these changes to which we 

 may here allude. It appears to be a phenomenon principally due to 

 pressure : as in the buried peat of Dartmoor, and the buried clays of 

 Holderness, the originally liijlit and uncoudensed matters have been 

 compressed to some considerable firmness, solidity, and weight; so 

 in respect of ancient clay?, mere pressure appear* to be a sufficient 

 cause for their condensed, often shaly consistence. But more than 

 this must be appealed to for the induration of slate, coal, limestone, 

 and sandstone. The particles of these rocks cohere with force, by a 

 process of internal molecular arrangement ; and it appears to be a 

 fair inference from numerous facts, that among the most influential 

 of the exciting causes of this molecular action is the internal heat of 

 the globe locally manifested in the induration of stratified masses 

 in the vicinity of trap dykes and other igneous rock-mosses, and 

 generally exhibited in the hardness, symmetrical structure, and 

 crystalloid aspect of the lower and more ancient strata, which may 

 be supposed to have been more subject, and for a longer time than 

 the others, to the influence of subterranean heat 



Very unequivocal signs of this action of heat appear in the vicinity 

 of the granite rocks of Cumberland, Devon, Cornwall, Wicklow, etc., 

 and perhaps they are seldom absent from such situations. One of 

 the most ordinary of such effects is the development, in the mass of 

 the stratified rocks, of peculiar crystals, a felspar near the granite of 

 Wastdale Head, in virtue of which slates become porphyry; of horn- 

 blende and chiaatolite in those of Skiddaw, just as garnets ore 

 developed in argillaceous beds near the trap rocks of Tecsdale and 

 Anglesey. [UocKS,] 



Some cases of re-arrangement of particles depend on molecular 

 action, not excited by heat, but determined to particular centres by 

 the previous existence there of solidified bodies. Oolite is an example 

 of common occurrence among limestone* of the secondary class, and 

 few things ore more interesting to examine than a polished slab of 

 the oolite of the mountain limestone of Bristol and Lancashire, or 

 the pisolite of Wilts. In the centre of the spherical grains of the 

 former, and in or near the axia of the less regular nodules of the 

 Utter, are grains of sand, bits of shells, or small foraniinifera, and 

 round these parallel coats of carbonate of lime ore neatly and con- 

 centrically arranged. An extremely large and irregular pisolitic 

 nodule gives us the link between these concretionary arrangement* 

 and the ' nodule* ' or ' balls ' of limestone and ironstone which inclose 

 pieces of plant*, leaflets of ferns, shells, bones, or fish-scales, ami lie 

 in parallel layers in the coal-shale*, lias-clays, &c. These balls are 

 evidently formed by accretion round tho organic object* which they 

 inclose, and the process is the more curious, because the matter of 

 the ball i* usually more calcareous or more ferruginous than the 

 surrounding matrix, and Menu to hare been collected from out of it 

 by some peculiar elective attraction depending on the nature of the 

 organic body. (Fig. 9). 



Re-arrangement of deposited nutter In nodules, round organic or inorganic 

 masses : t, one of these cracked as In Srjitaria. 



In addition to this process, it has frequently happened that the 

 ' ball ' ha* been cracked internally across by subsequent contraction 

 during consolidation, and the crack* are filled by carbonate of lime 



