Parr LJ - CONCRETIONS. 489 
occur in some sandstones, which, when exposed to the weather, 
decompose into Jarge round balls. In other instances, a fer- 
ruginous cement is gradually aggregated by percolating water in lines 
which curve round so as to enclose portions of the rock. These 
lines, owing to abstraction of iron from within the spheroid and 
partly from without, harden into dark crusts, inside of which the 
sandstone becomes quite bleached and soft." Some shales exhibit a 
concretionary structure in a still more striking manner, inasmuch as 
the concretions consist of the general mass of the laminated shale, 












































Fic. 206.—Ciay CoNcCRETIONS OF ALLUVIUM. (NAT. SIZE.) 
and the lines of stratification pass through them and mark them out 
distinctly as superinduced upon the rock. Examples of this structure 
are not infrequent among the argillaceous strata of the Carboniferous 
system. The concretionary olive-green shales and mudstones of 
the Ludlow group, in the Upper Silurian system, exhibit on weathered 
surfaces, all the way from South Wales into central Scotland, a 
peculiar structure which consists in the development of concentric 
spheroids varying from less than an inch up to several feet in diameter, 
the successive shells being separated from each other by a fine dark 
ferruginous film. The lines of stratification are sometimes well 
marked by layers of fossils, but the rock splits up mainly along 
the curved surfaces separating the concentric shells. Concretionary 
1 See Penning, Geol. Mag. Dec. 2, iii. May, 1876. 
