32 RECORDS OF THE AUSTRALIAN MUSEUM. 



The presence of Crinoids indicates an open fairly deep sea, 

 whilst the conglomerates, boulders, opalised saurians and wood 

 rather point to shallow water conditions with land at no great 

 distance. In the absence of exact knowledge as to the vertical 

 distribution of these fossils, it is idle to speculate on the geo- 

 graphical conditions obtaining at the time when the beds con- 

 taining these enigmatical specimens were laid down. The presence 

 oi gypsum is not conclusive, for gypsum may originate 

 either as a chemical deposit in an inland sea, or salt 

 lake, or, on the other hand, may be formed subsequently 

 to the deposition of the beds in which it occurs, for 

 example by the action of decomposing pyrites on calcareous 

 matter. According to Prof. J. D. Dana^ where gypsum occurs 

 not as continuous layers but in embedded, nodular masses, it was 

 formed after the beds were deposited. This criterion does not 

 help us to a conclusion, for Mr. Jaquet says^ that the gypsum 

 occurs both as isolated masses and as thin beds. In the recent 

 surface deposits of the western districts of New South Wales 

 gypsum is commonly met with as crystalline masses, where it is 

 undoubtedly of secondary origin and due to chemical interaction 

 between the constituents of the soil, and it is possible that a similar 

 origin is to be assigned to the gypsum found in the opal-bearing 

 beds. Against the likelihood of the gypsum being the result of 

 evaporation in a land-locked sea is the compaiutive abundance of 

 organic remains, for, when the water of an enclosed basin has 

 reached a degree of concentration that permits of the deposition 

 of gypsum from solution, animal life is usually absent. But it is 

 conceivable that a temporary lake may have been formed as a 

 remnant of a retreating ocean, and then subsequently re-united to 

 the waters of the Cretaceous sea. Any solution of the problem 

 presented by the pseudomorphs must be compatible with the pre- 

 sence of gypsum in the same beds. 



Both gypsum and the mineral now known to us only as opaline 

 casts have been converted into opal, the former partially, the 

 latter entirely, by the action of highly silicated springs to which 

 the general opalisation of the Desert Sandstone is usually 

 attributed. 



Previous Ohse7-vers. — The pseudomorphs were apjiarently first 

 observed by Jaquet,* by whom they wei-e referred probably to 



■'' Dana — Manual of Geolojry, 4th pd., 1895, p. -Doi. 

 ^ Jaquet — Loc. cit., p. 141. 

 ■' Jaquet — Lor cif., p. 141. 



