576 CASTS OF KADIOLAUIA IN PHE-CAMBlilAX KOCKS, 



(b) Crijslnl Brook. — In tli(3 black chert of Crystal Brook, the 

 radiolarian casts are chiefly in the form uf small spherical or 

 oval nuclei of chalcedon}^, with a more or less distinct j^artially 

 translucent outer ring of chalcedony. Much black opaque matter 

 is present in this rock, as well as small spherical developments of 

 iron pyrites, very suL'gestive of being inner casts of radiolaria. 



The Crystal Brook forms, as to the radiolarian character of 

 which we think there can be very little question, are shown on 

 Figs. 1-3 of PI. XXXIX. Their diameter varies from -1 mm. to 

 •2 mm. Figs. 1-3 are very suggestive of forms allied to Carpo- 

 sphcera. 



4. Geological Horizon of the Radiolarian Rock. 



As already stated, the two chief localities in South Australia 

 where the supposed radiolarian casts have been met with are (a) 

 Brighton and (6) Crystal Brook. These localities merit separate 

 descriptions. 



(«) Brighton. — The rocks from Brighton which have yielded 

 the casts aboA'e referred to were taken from the quarries of the 

 South Australian Portland Cement Comi^any, situated at Brighton, 

 about 10 miles S.S.W. from Adelaide, on a spur of the Mt. 

 Lofty Manges, which at this point describe a curve to the seashoi*e, 

 marking the southern Ijoundary of the Adelaide plains. 



The limestones worked by this company form outcrops rising 

 from beneath the Pliocene clays of the plain, aiid can be traced 

 for miles over the low hills to the south in a line almost parallel 

 to the coast. The workings extend at intervals for a distance of 

 about 200 yards across the outcrop, and about a quarter of a mile 

 along the line of strike. The succession of beds can be easily 

 traced, and is as follows, in descending order : — 



1. Buff-coloured Limestone. — The uppermost bed exposed in the 

 workings. It is very persistent and maintains its characteristics 

 for a long distance. Distinguished by its colour, contains a con- 

 siderable proportion of magnesium carbonate, is very tough and 

 hai'd. This bed is not quarried for cement, and marks the horizon 



