72 ORIGIN AND OCCURRENCES OF PHOSPHATE ROCK 



of the Cambrian and Silurian at Xerike in Westergotland, 

 Sweden. The overlying Silurian limestone is rich in 

 phosphatic nodules and glauconite, and almost everywhere 

 in Sweden it has been observed that the Lower Silurian 

 begins with a glauconitic bed with phosphatic concretions. 

 The phosphatic nodules are probably derived by the con- 

 cretionary action of water charged with phosphoric acid 

 from the decomposition of brachiopod beds containing 

 obolus and acrothele. There is always a break in the faunal 

 sequence where phosphatic nodules occur. The matrix 

 contains a mixture of two fanuas, due in part to weathering 

 by subgerial denudation. In the Cambrian Bala Beds of 

 North Wales it is trilobite remains which have given rise 

 to phosphatic concretions. In the New Brunswick deposits 

 we have sand and glauconite cemented by phosphate of 

 lime, and the nodules always contain trilobite remains, 

 sponge spicules, and protozoan tests. 



In the Liassic strata of Lorraine and of the Mendip 

 Hills, and in the Oxford clays phosphatic nodules occur. In 

 the latter stratum they contain casts of cei^halopods, 

 lamellibranchs, echinoderms, phosphatised sponges and 

 wood, concretionary masses, bones, teeth, coprolites of fishes 

 and saurians, and pebbles of phosphatised sandstone, and 

 compact phosphate. 



In the Devonian of Tennesse above the Chatanooga 

 Black Shale, there is also a nodular deposit containing 

 glauconite. 



These nodular deposits have been almost universally 

 ormed along former shore lines, where erosion of coral, 

 trilobite, molluscan and brachiopod limestone was going on 

 by subaerial agencies and phosphatic concretions were 

 washed out into the coastal muds just at or below low tide 

 limit. For this reason, too, the phos]3hate beds always 

 occur where beds thin out as well as where an unconformity 

 occurs. Thus the phosphatic bed overlying the Chatanooga 

 shales in Tennessee is only seven to eight feet thick, but in 

 Virginia the same bed is 400 feet thick and contains no 

 phosphatic nodules. 



Phosphates may also form by direct deposition in deep 

 water in association with glauconite. Some of the paloeozoic 

 and mesozoic beds of phosphatic nodules and glauconite 

 may have their origin. i 



