THE STRUCTURE OF ROCK MASSES. 
115 
ation to the true plane of sedimentary deposition : this is 
commonly called “cross or false” bedding, and is usually as- 
cribed to the disturbing action of currents during the period of 
the formation of the strata ; what has by Sorby been called 
“ ripple drift ” is an analogous structure. In the same mass of 
rock such false bedding may sometimes be seen inclined in 
opposite directions, in beds superposed one above another, 
a result probably due to alterations in the direction of the 
current in the same locality, such as the ebb and flow of the 
tides. 
In most cases the planes of stratification in large rock masses 
can be traced with ease, owing to the occurrence of alternating 
beds differing in mineral character, such as those of argillaceous, 
arenaceous, or calcareous nature ; when fossiliferous, the organic 
remains are a sure guide, since they naturally rest upon the 
planes of bedding which formerly were the sea bottom, and 
when such anciently formed the margin or shores of seas or 
lakes, it is not uncommon to find ripple marks, foot-tracks of 
animals, marks after rain drops, and cracks formed by the heat 
of the sun, causing the mud to contract in drying between the 
reflux and influx of the tides, all of which are so many proofs of 
the origin of this structure. Certain strata, such as the lime- 
stones, the chalk with its flint nodules and bands, the infusorial 
siliceous beds, and some of the iron-stone beds, have assumed 
the form of sedimentary strata, not by the mere mechanical 
deposition of mineral matter held temporally in suspension in 
the water, but by the agency of organic life in extracting the 
lime, silica, or iron, previously held in a different state of 
chemical combination and solution in sea or fresh water : 
abundant examples of this action still going on before our eyes 
are seen in the formation of the great coral reefs, the Atlantic 
mud, which is but recent chalk, and the lake iron-stones, which 
last have only lately attracted any attention, and are as yet very 
imperfectly described. 
Other strata, as for instance the beds of rock salt, gypsum, 
and anhydrite, have no doubt been originally formed simply 
by the evaporation of inland seas or lagoons of salt water, pre- 
viously holding these substances in solution. 
In some rocks, more especially those pertaining to the older 
formations, in which, as yet, few or no fossil remains have been 
found, it is often a matter of considerable difficulty to deter- 
mine correctly the lines of original sedimentary deposition. In 
such cases, dependence must be placed mainly on the observa- 
tion of such minor differences in mineral character and structure 
as usually present themselves, even in the thicker masses of 
sedimentary rocks, and in the case of extremely homogeneous 
deposits, such as, for example, some of the great beds of clay- 
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