Page:A Treatise on Geology, volume 1.djvu/213

 CHAP. VI. even very red. In texture, a few limestones are compact, some oolitic, many cellular, the cells lined with crystallised carbonate of lime, a large proportion of a fine sandy grain, some quite powdery, with crystallised balls included; and in Nottinghamshire, considerable tracts yield granular crystallised limestones. Near Sunderland laminated rocks are really of sparry texture. Strings and plates of spar are very common, and render buildings of the magnesian limestone very irregular in their decay, from the unequal perishing of the stone between the ribs of spar.

Structures of Deposition.—Stratification is distinct in all these rocks; but in all of them some peculiarities appear in this respect.

The fine-grained upper limestones of Knottingley are thin-bedded: the granular rocks of Nottinghamshire are either thick-bedded or flag-like; it is sometimes difficult to trace the beds at all in the powdery magnesian rocks; and in certain sparry rocks near Sunderland, the bedded structure is almost overlooked in admiration of the coralloidal forms of the concretionary masses, which sometimes are enveloped in soft yellow powder (Building Hill).

Divisional Planes.—The fine-grained limestones of Knottingley are traversed by vertical divisions from top to bottom, which in some places are open to a foot in width, or filled with clay and rolled pebbles; in other cases they are merely thin cuts in the rock; always their regularity, parallelism, and polarity (if we may so term their direction to N. or N. N. W., and its rectangle E. or E. N. E.), are remarkable. In other thick bedded limestones, the joints are less symmetrical, though always numerous: most of the rocks are traversed by small secret cracks, which, on being exposed by fracture, are found covered by dendritical markings of a dark colour. The joints are often coated by carbonate of lime, sometimes by carbonate of copper, or sulphuret of lead.

Succession and Thickness of Strata.—The most, or