74 ORIGIN AND OCCURRENCES OF PHOSPHATE ROCK 



markably rich in life — molluscan, pteropod, brachiopod, 

 trilobite, coral and foraminiferal remains constituting large 

 limestone formations. Thus the beds of the Devonian lime- 

 stone of the Currockbilly Range of New South Wales are 

 almost wholly composed of brachiopods. Where such 

 formations are followed by an unconformity, and con- 

 temporaneous erosion of the deposit has taken place, rich 

 pockets of phosphorite are possible. An unconformity 

 would be recognised by the prospector if he sees a change in 

 the dip of the overlying strata from that of the limestone, 

 and notices at the same time that immediately above the 

 latter formation comes a bed of coarse shingle. 



In these paloeozoic limestones glauconitic phosphate rock 

 of deep-sea deposition might be met with. Such deposits 

 would probably be of a dark colour, due to manganese 

 staining and might contain remains of deep-sea trilobites 

 (blind forms), cephalopods, pteropods, and sponges with 

 micro-organisms belonging to the foraminifera, radiolaria, 

 infusoria, and diatoms. 



It now behoves us to consider if in Australia we may 

 anywhere expect a counterpart of the Florida phosphates. 

 In Florida we had an elevation of Tertiary limestone deposits 

 accompanied by drying up of salt marshes and by an inroad 

 of mammals driven south by the great Ice Age. As the 

 marshes dried up and the cold increased the animals died 

 in large numbers of starvation and left their bones in caves 

 and solution fissures of the limestone. Bats and birds 

 preying upon the carrion left their guano in caves and 

 fissures. The limestone, itself rich in phosphoric acid from 

 the high proportion of foraminifera in it, was further enriched 

 in that ingredient by leaching out of carbonate of lime. 



In Australia we have had no great universal Ice Age 

 in Tertiary times, but for all that Ave have had an almost 

 equal extermination of animal life by the desiccation of the 

 now arid interior. It is believed by Austrahan geologists 

 that until the Pliocene or early Pleistocene, large stretches of 

 the desert interior were covered with vast lakes. Where 

 the land now lies scorched and parched the rain then fell 

 in copious amount. Vast herds of giant marsupials Hved 

 around these lakes and along the streams that watered them. 

 Granually the climate got drier and drier. The animals of 

 the Australian Interior would as a result be scattered in 



