70 



Again, in this case, also, no definite molecular compo- 

 sition meeting both the petrological and chemical require- 

 ments can be assigned. 



The salient fact demonstrated by the analyses is that, in 

 the case of the first mineral, the light yellov; encrusting iso- 

 tropic phosphate is a mineral of the evansite type ; the other 

 variety, found in globular aggregates, and appearing aniso- 

 tropic, is distinctly of the beraunite type, perhaps closely 

 related to globosite. 



VI. Summary. 



1. Elder Rock is a prominent, abrupt mass, standing in a 

 level plain in the arid northern districts of South Australia. 



2. A hard yellow phosphate encrusting the top of the 

 rock is the remnant and more insoluble part of a larger 

 deposit of guano, chiefly bird-derived. 



3. The soluble salts of the guano have, to some extent, 

 saturated the rock for a depth below; in a favourable spot 

 this impregnation has been effective for at least 46 feet in 

 the vertical. These compounds are chiefly nitrates of am- 

 monia, with some fixed alkali and phosphate. The more 

 soluble compounds have percolated to greater depths. 



4. The hard yellow phosphate on top of the rock is a 

 mixture of minerals, chiefly iron and aluminium phosphates, 

 and has no definite composition ; it is, however, here referred 

 to as paratooite. Two widely-divergent types are described, 

 the commoner near to evansite^ and a scarcer globular phos- 

 phate related to beraunite. 



5. Paratooite probably has a wide distribution, as small 

 scattered occurrences in the arid northern parts of South 

 Australia. 



So far as we are aware, no similar phosphate has ever 

 previously been described from Australia. 



DESCRIPTION OF PLATE XX., FIGURE 2. 



Elder Rock, seen from the east; barren saltlnisb plains in 

 the foreground. 



