SEPT ARIA, PHOSPHATITE, AND LIMESTONE. 47 



Septaria. Clays also contain concretions formed of a mixture of 

 lime and clay. They are often oblate elliptical spheroids, and are 

 named septaria. In new strata they are very small, and may be seen 

 in process of first formation in little nests in the brick-earth of the 

 valley of the Thames, as at Crayford. They are found in nearly all 

 clays, and are larger in the older rocks, showing that they grow gra- 

 dually by gathering to themselves, by the solvent action of water, the 

 lime which the clay contains. In the London Clay they are usually a 

 foot or two in diameter ; and in most of the clays are under six feet 

 across. Brickmakers often call them turtle-stones. In the Ludlow 

 rocks they are called ball-stones, and are sometimes eighty feet in 

 diameter. When burned and ground to powder these concretions form 

 hydraulic cement, which sets under water. Septaria are so named 

 from the partitions or septa by which they are divided. They owe 

 their existence to the fact that while the clay was forming, lime also 

 was being thrown down upon the sea-bed, but in quantity too small to 

 form continuous beds of limestone. And proof of this is seen in the 

 fact that layers of septaria, which cover the clay-floor much as raisins 

 might cover a surface of dough, may sometimes be traced into more 

 or less continuous beds of rock. These concretions are rarely or never 

 met with when the clay is sandy. See p. 102. 



Phosphatite. Occasionally beds of small concretions of phos- 

 phate of lime, sometimes called coprolites, rest on clay surfaces or are 

 scattered in sands or limestones. They are highly valued for the 

 manufacture of an artificial manure for root-crops, which is named 

 superphosphate of lime. The chief phosphatic beds in this country 

 are six in number : in the Bala series of North Wales, in the Upper 

 Neocomian, Gault, Upper Greensand, Coralline Crag, and the Red 

 Crag of the south-east of, England. These deposits appear to have 

 been owing chiefly to the growth and decay of sea-plants for many 

 generations, on fixed spots near to the shore, since those plants all 

 contain a quantity of phosphates, which are capable of combining with 

 lime when liberated by the decay of their organic tissues. These con- 

 cretions rarely assume a septarian structure ; and the mineral often 

 invests or infiltrates animal substances. See p. 103. 



Limestone. Limestones consist of the mineral calcite, though 

 usually in an uncrystallised condition. River waters carry dissolved a 

 good deal of carbonate of lime, and Sir Charles Lyell has recorded that 

 the evaporation of the waters of the Rhone which float over the sea is 

 forming a calcareous rock off its delta, in which sunken cannon had 

 become embedded. At the base of chalk cliffs the chalk is always 

 rounded into large boulders, as may be well observed at the Culver 

 Cliff on the east of the Isle of Wight ; and the peculiar green colour 

 of the sea-water off chalk coasts has been attributed to the particles 

 of abraded chalk which it contains. Such particles may be presumed 

 to form a deposit in which small chalk pebbles occur ; for a recon- 

 structed limestone bed of this kind may be seen in the geological 

 deposit called the Stonesfield slate. 



Oolite Another group of limestones is the oolites, so called from 



