316 



colour (pis. zvii. and xviii.), cross jointed, very brittle, and 

 under percussion break up into numerous small pieces. There 

 has evidently been some molecular reconstruction as the 

 siliceous constituent is, under normal conditions, distributed 

 throughout the mass, but at the Burr a it is, to some extent, 

 segregated into layers and nodules of black cherts, by which 

 the limestone becomes relatively purer and crystalline. The 

 development of black chert in the lim-estone gradually 

 increases along the strike, from north-west to south-east, until 

 at the New Burra Mine, about nine miles from Kooringa, the 

 cherty layers in the limestone are very numerous. 



It is open to question as to whether this unusual 

 development of laminae in the limestone, in this particular 

 district, arises partly from the development of an incipient 

 foliation under pressure. It is evident that there has been 

 some measure of metamorphic reconstruction, and while there 

 is, generally, a remarkable regularity in the parallelism of 

 the layers, the lines sometimes become contorted, broken^ 

 wavy, and even concentric (pi. xviii.). 



Pekina Creek. 



A limestone of the same horizon as the Brighton beds 

 outcrops on the Pekina Creek, near Orroroo (176 miles, by 

 rail, north of Adelaide), and shows a slight variation of 

 structure from that which occurs at Brighton. The ground- 

 mass, as seen through the microscope, consists of distinct and 

 clear crystals of calcium carbonate, that are more perfectly 

 developed than at Brighton. In this groundmass are set a 

 number of objects of darker hue than the crystalline matrix. 

 They are of various shapes^ — circular, quadrate, triangular, 

 fusiform, mushroom-shaped, and irregular, of various sizes up 

 to one-eighth of an inch in diameter. The contours, by which 

 these bodies are defined, are usually rounded and strongly 

 marked. The bodies are composed of granular particles of 

 limestone of very fine texture and, towards the outer margin, 

 the granules are sometimes arranged in a lineal and concentric 

 structure that simulates very fine organic cellular structure. 

 The central areas do not show this regular arrangement of 

 the granules, and are usually more open in texture and pass 

 transmitted light more readily than the marginal portions. 

 The general texture and appearance of the rock are closely 

 similar to the Brighton beds, but the darker coloured bodies 

 in the clear matrix are of larger size and more irregular in 

 shape than those seen at Brighton, but they are evidently 

 of a similar kind and have had a common origin. 



