THE OCEAN AND ITS WORK 239 



Although lime-secreting organisms are found at all 

 depths of the ocean, yet the most important and abundant are not 

 found at depths greater than light can penetrate (p. 197). 



Mud and sand are mechanical or clastic (Greek, clastos, broken) 

 sediments; that is, they are derived from the decay of rocks and are 

 brought directly to the sea by streams or by waves. All of the 

 calcium carbonate which has accumulated to form limestone was, 

 on the other hand, brought to the ocean in solution. Some'of it was 

 precipitated directly from the water, since salt water is capfble of 

 holding a smaller quantity of calcium carbonate in solution than 

 fresh water. The massive gray submerged limestone off the south 

 coast of England contains modern shells, proving that precipitation 

 is now taking place. Calcium carbonate is abo precipitated by the 

 ammonium carbonate derived from the decay of organisms. 



The most common limestones are formed from the accumulations 

 of the remains of mollusks, corals, sea urchins, starfish, crinoids 

 (p. 430), Foraminifera (p. 523), and other marine animals, and of cer- 

 tain plants (calcareous algae). One sometimes sees ledges of limestone 

 almost completely made up of a jumble of shells of one or two species 

 of mollusks. Limestone often shades imperceptibly into shale or 

 fine sand. 



We consequently find in ancient rocks that conglomerates (p. 249) 

 usually occur in relatively narrow belts, and sandstones often cover 

 wide areas, while shales and limestones have a still wider distribution, 



Lens-shaped Sediments. — All sedimentary deposits are roughly 

 lens-shaped. They are thickest as well as coarsest near the 

 source of supply, and become finer and thinner away from it. This is 

 most noticeable in conglomerates (p. 249) which in a distance of even 

 two or three miles may decrease from a thickness of perhaps several 

 hundred feet to that of a few feet, or may disappear entirely. Sand- 

 stones have a similar character, but usually thin out much less rapidly ; 

 while muds, or their equivalents, shale and limestone, may extend 

 many miles with slight variation in thickness. Beds of limestone 

 only a few feet thick can sometimes be traced over an area of several 

 ^hundred square miles. 



Dovetailing of Sediments. — If a boring were made a few miles 

 from shore, through sediments which had accumulated to a consider- 

 able thickness, it would seldom penetrate a single kind of rock for a 

 great depth, but would, for example, first pass through sandstone, 

 then shale, then sandstone again, and perhaps through limestone. 



CLELAND GEOL. — 1 6 



