490 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. _ 
structures are found also in rocks formed from chemical precipitation, 
as for instance in beds of rock-salt. The pseudo-concretions probably 
due to pressure (stylolites) have been already described (p. 313). 

Fic. 207.—ConcrETIONARY STRUCTURE IN Upper SiLurtaAN SHALes, Cwm-Dpv, 
LLANGAMMARCH, BRECKNOCKSHIRE (B.). 
Alternations and Associations of Strata.—Though great 
variations occur in the nature of the strata composing a mass of 
sedimentary rocks, it may often be observed that certain repetitions 
occur. Sandstones, for example, are found to be interleaved with 
shale above, and then to pass into shale; the latter may in turn 
become sandy at the top and be finally covered by sandstone, or. 
may assume a calcareous character and pass up into limestone. 
Such alternations bring before us the conditions under which the 
sedimentation took place. A sandstone group indicates water of 
comparatively little depth, moved by changing currents, bringing 
the sand now from one side now from another. The passage of such 
a group into one of shale points to a diminution in the motion and 
transporting power of the water, perhaps to a sinking of the tract, 
so that only fine mud was intermittently brought into it. The 
advent of limestone above the shale serves to show that the water 
cleared, owing to a deflection of the sediment-carrying currents, or to 
continued and perhaps more rapid subsidence, and that foraminifera, 
corals, crinoids, mollusca, or other lime-secreting organisms, estab- 
lished themselves upon the spot. Shale overlying the limestone 
would tell of fresh inroads of mud, which destroyed the animal life 
that had been flourishing on the bottom; while a return of sandstone 
beds would mark how, in the course of time, the original conditions 
of troubled currents and shifting sandbanks returned. Such alterna- 
ting groups of sandy, calcareous, and argillaceous strata are well 
illustrated among the Jurassic formations of England (Fig. 208). 
Jertain kinds of strata commonly oceur together, because the 
conditions under which they were formed were apt to arise in succes- 
sion. One of the most familiar examples is the association of coal 
and fire-clay. A seam of coal is almost invariably found to lie on a 
bed of fire-clay, or on some argillaceous stratum. The reason of this 
union becomes at once apparent when we learn that the fire-clay 
was the soil on which the plants grew that went to form the coal. 
Where the clay was laid down under suitable circumstances, yegeta- 

