The Lower Ordovician Succession in Scandinavia. 655 



type of Alum-Shales of this division by the sandy beds with Obolus 

 iii Northern Oland and the Baltic Provinces is discussed, and is con- 

 sidered as an early manifestation of the crustal instability which 

 brought about the curious vicissitudes of the Glauconite- Shale 

 period. Many of the phenomena at the base of Division B are 

 suggestive of an unconformity, and in all Swedish areas except 

 Skane the succession from A to B is shown to be incomplete. In 

 general, the base of Division B has much the aspect of the Cam- 

 bridge Greensand, and the Glariconite- Shales as a whole mark a 

 period during which the alternating processes of deposition and 

 erosion were nicely balanced. A curious small-scale erosion- 

 phenomenon occurring intermittently throughout Division B is 

 redescribed under the name ' Korrosionsgrupper,' and along with 

 the variable detailed succession of the beds is considered as a 

 further proof of discontinuous sedimentation throughout the period. 

 Division C of Didymograptus-Shales and their equivalent Orthocera- 

 kalk is discussed only in outline. Both shales and limestone pass 

 downwards continuously into the highest Cer 'atopy Y/e-Beds, both in 

 lithology and fauna, and the difference between the lowest Orilio- 

 ceras-Limestone (Planilimbatenkalk) and the higher limestone of 

 the Ceratojiygekd^k is almost imperceptible. 



This stratigraphical evidence is then considered in its bearing upon 

 the question of the definition of the boundary between the Cambrian 

 and the Ordovician Systems ; and the author follows the Scandinavian 

 authorities in considering that, so long as the Dictyonema -horizon 

 is available, the evidence of sudden faunistic change within the 

 series discussed is too slight to warrant a palaeontological separation 

 of the systems at any other horizon. A comparison of the British 

 Tremadoc and Arenig Series with these Scandinavian rocks con- 

 cludes the paper ; and it is maintained that the time has now arrived 

 for British geologists to come into line with their Continental 

 brethren, and to include the Dictyonema- and the overlying Tremadoc 

 Beds as the lowest series of the Ordovician System. 



2. ' The Occurrence of Pseudomorphous Pebbles of Pyrites at 

 the Crown-Beef Mine (Witwatersrand).' By Cuthbert Baring 

 Horwood, A.R.S.M., Assoc.M.Inst.C.E., P.G.S. 



Reference is first made to the existence of calcite-' pebbles ' in 

 the Main Reef, which Mr. Julius Kuntz believes to be due to the 

 replacement of quartz by calcite. Pellets of iron-bisulphide known 

 as ' buckshot ' occur at the Rietfontein ' A ' Mine in the Buckshot 

 Reef : they exhibit radiate fibrous structure, and are probably of 

 concretionary origin. At the Crown-Reef Mine a few l pebbles ' 

 of pyrites, some measuring as much as an inch in length, occur in 

 a narrow band of conglomerate at the contact of the reef with a 

 basic dyke. It is conjectured that the mineralizing solutions, 

 which deposited the pyrites (together with, some if not, all of the 

 associated gold) ascended along the fractures due to the intrusion 

 of the dyke, and found an easy course along the small conglomerate- 

 bed, where they replaced some of the quartz-pebbles with pyrites, 

 being kept up by a band of shale underneath the conglomerate. 



