EPICONTINENTAL REEFS 403 



depth from ten to twenty-five fathoms, and has a sandy bottom. 

 Southward, where the reef increases in distance from the shore, 

 the depth of the lagoon increases to forty and in some places more 

 than sixty fathoms. Just outside of the reef the water descends 

 to profound depths. 



The surface of the reef consists of a hard, white agglomerate of 

 many kinds of corals with numerous projecting points. The outer 

 rim is highest, and is traversed by narrow gullies, and is breached 

 at rare intervals by ship canals. 



In form the Great Barrier Reef is a triangular wedge, widening 

 gradually from the shore outward for a distance of from 30 to 90 

 miles. Just outside of the reef the descent to abyssal depths is 

 abrupt, the outer face of the reef forming a great wall in some cases 

 exceeding 1,800 feet in height, and fronting the whole northeast 

 coast of Australia. Along the outer edge of the platform produced 

 by the reefs is the true barrier, a linear series of reefs breached by 

 narrow passages. Inside the barrier is a clear and broad channel 

 generally from 15 to 20 fathoms deep, with its bottom covered by 

 unconsolidated lime sand or by a sand largely composed of the 

 foraminiferan Orbitolites, these remains in some cases also making 

 up the whole sand of the beach either of the coral islets or the neigh- 

 boring shores. Outside, in the deep sea, the dredge obtained only 

 a fine-grained, impalpable, pale, olive-green mud, which was wholly 

 soluble in dilute hydrochloric acid, and when dried had the charac- 

 ter and consistency of chalk. 



Great masses of Porites and M?eandrina flourish on the outer 

 border of the reef, and these when detached from their anchorage 

 are rolled about by the waves and worn, while at the same time 

 grinding down the rock mass beneath until the whole coral head is 

 reduced to fine sand or mud. Such rounded detached masses of 

 IMaeandrina 6 or 8 feet in diameter are common among the loose 

 blocks rolled up from the outer slope onto the reefr They may be 

 seen just inside the surf at low water, extending for some miles, 

 and are known as "Turks' heads." Madrepora, Millepora and 

 other genera also characterize the outer reef together with numer- 

 ous species of astrseans. Inside the channel mentioned lie the "in- 

 ner reefs" separated from each other by narrow waterways through 

 which the tide rushes with great force. Sometimes these tidal cur- 

 rents continue in the same direction even for two or three days, es- 

 pecially after great storms have driven large volumes of water into 

 the reef area. Between the inner reefs and the mainland of old rock 

 lies a shallow channel, mostly free from coral growth. 



Among the inner reefs are many, which are to a large extent 



