GEOLOGICAL SEQUENCE OF THE DOWNING BEDS. 293 



have played an important part. Above the limestone is the Lower 

 Trilobite bed of shale, sandstone in part, probably seventy feet 

 thick. A very impure limestone, about twenty feet thick, is the 

 next in ascending order, and from the prevalence of trilobites, T 

 have designated it the Middle Trilobite bed. From the great 

 preponderance of the genus rhacojys, Mr. C. Jenkins, L.S., in his 

 paper on the Geology of Yass Plains, denominated it tlie 

 " Pliacops Bed." 



Above it lies the great shale bed, which I estimate to have a 

 thickness of 1,300 feet to 1,500 feet. In the lower portion thin 

 bands of flaggy sandstones are found, and the same feature is 

 present towards its close. In the eastern half of the syncline I 

 have not found any fossils in this bed, though throughout markings 

 that resemble seaweed prints are observable. On the western side, 

 however, I have obtained small Brachiopods and Gasteropods, 

 from the upper strata. It would appear from the absence of 

 organic remains that it must have been formed under a deep sea. 

 After the completion of this bed, conglomerates, sandstones (some 

 flagstones), and shales, repeat themselves not less than three times, 

 pi'oducing a total thickness that cannot be less than seven hundred 

 feet. Some of these later shales are very soft and laminated, and 

 the shales throughout would largely come under the designation of 

 mudstones. Crowning the series is a thick bed of a remarkable 

 conglomerate, compact, hard, and in part metamorphic. The whole 

 thickness of the eastern section I estimate at 2,000 feet to 2,500 

 feet. 



The western section of the syncline at the base, includes some 

 grits, friable sandstones and shales, which are not found in the 

 eastern half. These appear to have a thickness exceeding seven 

 hundred feet. Therefore, the sedimentary rocks in this sectionmust 

 nearly, if not, be quite 3,000 feet in vertical measurement. 



In this portion also, some of the beds have undergone a greater 

 degree of metamorphism. The limestone which has diminished in 

 thickness, has become silicified, and were it not for the fossils 

 could not be recognised. 



From the porphyry ridge on the west side of syncline to Flinter's 

 or Conroy's Gap, the beds enumerated as occurring in the syncline, 

 appear to be repeated, but as extensive metamorphism has taken 

 place in them they are not distinguishable. 



Besides that agreement of the Bowning Beds with Palaeozoic 

 rocks of other parts of tlie world., in the ternary charater of 

 arrangement, the shales possess some features common to the 

 Silurian rocks of Britain. One of these is the tendency to form 

 into concretionary masses of concentric rings. Anotlier is the 

 distribution of nodular masses of lime through certain of them. 

 In some cases these nodules are in general of a spherical form, and 

 enclose fossils. Occasionally they occur in layers covering con- 

 siderable areas. The upper layer of these nodular masses is found 

 in the Upper Trilobite bed. 



