aye 


themselves by means of fibrous 
Parr L] CONCRETIONS. 487 
heaps associated with worm burrows, connected with pipes descend- 
ing into the rock, nor composed of different material from the sur- 
rounding sandstone or shale. ‘These may be conjectured to be due 
to the intermittent escape of gas from decomposing organic matter 
in ,the original sand or mud, as we may sometimes witness in 
operation among the mud flats of rivers and estuaries, where much 
organic matter is decomposing among the sediment. On a small 
scale these protrusions of the upper surface of a deposit may be 
compared with the mud-lumps at the mouths of the Mississippi, 
already described (p. 386). 
Concretions.—Many sedimentary rocks, more particularly clays, 
ironstones, and limestones, exhibit a concretionary structure. This 
arrangement may be part of the original sedimentation, or may be due 
to subsequent segregation from decomposition round a centre. Con- 
cretionary structures of contemporaneous origin, particularly in 
calcareous materials, may lie so closely adjacent as to form con- 
tinuous or nearly continuous beds (Fig. 202). The magnesian 











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Fic. 202.—SxrcrioN oF ALTERNATIONS OF SHALE AND CONCRETIONARY LIMESTONE (B.). 
limestone of Durham is built wp of variously shaped concretionary 
masses, sometimes like cannon-balls, grape-shot, or bunches of coral. 
Connected with concretionary beds 
are the seams of gypsum, which 
may occasionally be observed to 
send out veins into other gypsum 
beds above and below them. De 
la Beche describes a section at 
Watchet, Somersetshire, where, amid 

SRS ; ~~ Kia. 203.—SEcTION oF BEDS AND CON- 
the old Triassic marls (6 b in Fig. NECTING STRINGS oF GyPsuM IN 
203), beds of gypsum (a a) connect THE Trias, WatTcHET, SOMERSET- 
SHIRE (B.). 
veins with the overlying and underlying beds. 
The most frequent form of concretions is that of isolated spherical, 
elliptical, or variously shaped nodules, disposed in certain layers 
