Reports and Proceedings—Geological Society of London. 187 
that the branching graptolites of Division A show a progressive 
evolutional divergence from an original Dictyonema through Dictyo- 
graptus flabelliformis to the true graptolites of Clonograptid or 
Bryograptid types, and through D. norvegicus to the so-called 
‘dendroid graptolites.’ The gradual replacement of the western 
type of Alum, Shales of this division by the sandy beds with Obolus 
in 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 Glauconite 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 Ceratopyge Beds, both in 
lithology and fauna, and the difference between the lowest Ortho- 
ceras Limestone (Planilimbatenkalk) and the higher limestone of 
the Ceratopygekalk 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 paleontological separation 
of the systems at any other horizon. A comparison of the British 
Tremadoc and Arenig Series with these Scandinavian rocks con- 
eludes 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 Dietyonema and the overlying Tremadoc 
Beds as the lowest series of the Ordovician System. 
2. ““The occurrence of Pseudomorphous Pebbles of Pyrites at 
the Crown Reef Mine (Witwatersrand).”” By Cuthbert Baring 
Horwood, A.R.S.M., Assoc. M.Inst.C. E., F.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 
Réef: they exhibit radiate fibrous structure, and are probably of 
concretionary origin. At the Crown Reef Mine a few ‘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 mineralising solutions 
which deposited the pyrites (together with some, if not all, of the 
