326 Voyayes of Discovery and Survey. 



In the examination of Heron Island, the coral beds, 1 to 2 feet 

 thick, were observed by Hr Beete Jukes to have a tendency to split 

 into slabs, and joints were found to cross each other at right angles, 

 parallel to the dip* and strike respectively, dividing up the- coral 

 rock into blocks of 1 or 2 feet in the side. This jointing of an ac- 

 cumulation, forming as it were under the eye, has no small geological 

 bearing. 



It is interesting to consider the accumulations now collecting for 

 1000 miles inside the outer ridge of the great barrier reef. The 

 surf-loving corals probably extend over broken or triturated coral out- 

 wards, slowly advancing the main mass. While great depths bound 

 the outer edge, the inside becomes filled up by a multitude of corals, 

 which can thei-e adjust themselves to the needful conditions ; and by 

 coral sand-banks, which Mr Beetes Jukes shews are but " the wash- 

 ino-s of broken coral, swept by tides and winds towards the lee-side 

 of the reef, until that is made the shallowest." " When this is dry 

 at low-water, the sand is piled up by the wind into a heap, with a 

 sloping bank, till it is at last reared above high-water mark." 



Amid these corals and sand-banks, numbers of molluscs, radiata, 

 and fishes live, and at death leave a large proportion of their harder 

 parts, adding to the general accumulation, and often doubtless in a 

 stratiform manner. Nor are these the only classes of animal life, 

 the remains of which aid in increasing the general mass. Turtles 

 frequent the reefs, banks, and islets, where their skeletons and bones 

 are found scattered, and the conditions are such that these remains 

 can scarcely but be entombed amid the calcareous sands and coral 

 growth. Their eggs are known to be so imbedded. The bones of 

 birds also may propably be enveloped, for immense flights of them 

 are seen on the islets. Raine's Islet, mentioned as not more than 

 1000 yards long and 500 yards wide, and only 20 feet above the 

 sea, was found covered with them. 



The conditions for the solution and chemical deposit of carbonate 

 of lime would often arise, and a slight to-and-fro motion from a rip- 

 ple in sheltered situations, where carbonate of lime was being thrown 

 down from solution, would so roll about fine grains that they became 

 covered by concentric concretions, forming oolites. Mr Beete Jukes 

 found such upon Raine's Islet. Where circumstances were favour- 

 able the crystalline aiTangement would be upon a larger scale, and 

 definite forms would be presented. Our author found drusy cavities 

 in coral rock containing crystals of carbonate of lime, as also some of 

 sulphate of lime. 



While this order of accumulation was progressing from the clean 

 sea and outward reefs inwards, different conditions would obtain along 

 the shore of the mainland. It is well known to those who have 



* The dip was from 8° to 10°. 



