PEINCIPAL PHOSPHATE DEPOSITS. 37 



nodules, in the foliated and generally highly gypsiferous marls 

 which alternate with limestone beds. Teeth and debris of saurians 

 and fish are found therein. These marls are unctuous and greasy 

 to the touch, and contain as much as 7 to 8 per cent of organic 

 matter still badly examined, but which neither dissolves in carbon 

 disulphide nor in benzine. These foliated marls often pass to a 

 nodular structure. They contain irregular layers of phosphatic 

 nodules which occur in various sizes and shapes, generally rounded 

 or striated, covered with a shining brown crust of characteristic 

 appearance. The large nodules which are met with in these con- 

 ditions, and which have the appearance of enormous coprolites, 

 are on the contrary very poor. The phosphate is entirely concen- 

 trated in the shining surface crust, the interior is limestone. The 

 small nodules of identical appearance, on the other hand, have a 

 content which may reach 70 per cent of tribasic. The phosphatic 

 marls contain besides their phosphates, small interstratified crystals 

 of gypsum, nodules of celestine, strontium sulphate, and alkaline 

 salts. 



PJiospliatic Limestones. — These phosphates alternate with the 

 nodular marl. They occur as a granular, rather friable rock, the 

 colour of which varies from a yellowish-grey to a greenish-brown. 

 The quality most appreciated crushes easily between the fingers, 

 and its density does not exceed 2 for the rock in situ. This rock is 

 formed by the agglomeration into a more or less calcareous cement 

 by manv fine grains of all shapes ; some round, covered with a 

 brown, brilliant crust, consist essentially of yellowish phosphate of 

 hme, with an earthy fracture, or of fibrous appearance ; the others, 

 grass green, of scaly texture or in very small masses, of a scori- 

 aceous or corroded appearance, recall, by their aspect, certain 

 glauconites. There also occur very small grains of hyaline angular 

 quartz, and chemical analysis always shows in this rock an ap- 

 preciable amount of free or hydrated gelatinous silica ; finally, it is 

 also very rich in organic debris, such as coprolites, similar to those 

 of the foliated marls, teeth and fish bones, or of saurians, more 

 or less disintegrated. When the calcareous element predomin- 

 ates in this rock, it becomes greyish, and much resembles the grey 

 chalk (calcareous tufa) of Ciply. .These bands of phosphatic lime- 

 stone are extensive, and occupy a very variable position in the 

 different deposits, but very constant in each of them. Their thick- 

 ness varies from only a few centimetres up to three metres and 

 more, which they maintain without interruption over distances of 

 50 to 60 km. (western chain of Gafsa). The deposits may be 

 divided into four main groups. 1. Those of the Tebessa district; 

 2. Those of the district of Setif ; 3. Those of the Guelma district ; 

 4. Those of the Ain-Beida district. They contain 24 to 30-5 per 

 cent of phosphoric acid. 



