434 Geological Society. 



They foiiud that the amount of free disseminated silica increases 

 in proportion to the numher of spicules and calcite-casts of spicules 

 which occur in the rock, and ohserved that the great similarity be- 

 tween the siliceous chalk and the Malmstone was heightened by the 

 occurrence of similar siliceous concretions in both rocks, the material 

 of which might be described as siliceous chalk, indurated by a cement 

 of chalcedonic silica. The conditions in which the silica was found 

 in the Lower Chalk were described, in examples varying from those 

 containing least to those which held most silica ; in the latter the 

 amount of colloid silica was estimated at 12-61 per cent, by weight. 

 After noticing the vast amount of silica present in rocks with a 

 maximum thickness of 70-80 feet, the authors discussed the diffi- 

 culty of accounting for this, and drew attention to Prof. Sollas's 

 statement that many living siliceous sponges constantly shed some 

 of their spicules. 



A further question arose as to whether the formation and accu- 

 mulation of globular silica went on contemporaneously with the 

 deposition of the calcareous material upon the sea-floor, or whether 

 the conversion of the spicules into such silica took place after the 

 consolidation of the rock, and the authors gave reasons for sup- 

 posing that the latter was the case, the change having occurred 

 wheii the rock was in a sufficiently oozy condition to admit of easy 

 molecular distribution, lleasons were given for supposing that the 

 disseminated colloid silica had not been derived directly from the 

 disintegration of spicules in which a globular structure had been 

 previously developed, but that the globular silica was precipitated 

 from solution whilst the beds were still permeated by sea- water. 



The precipitation of the chalcedonic silica was regarded by the 

 authors as a secondary and subsequent operation. They were dis- 

 posed to regard all nodular concretions resembling flints and phos- 

 phatic nodules as growths, which were more or less contemporaneous 

 with the deposition of the materials of the enclosing rock, and in 

 conclusion they offered some comments upon the problem of the 

 formation of flints. 



3. "Note on the Pelvis of Oniithopsis." By Prof. H. G, Seeley, 

 r,ll.S., F.G.S, 



April 3.— W, T. Elanford, LL.D,, r,R.S,, President, 

 in the Chair. 



The following communications were read : — 



1, " The Elvans and Volcanic Rocks of Dartmoor," By E. N. 

 Worth, Esq,, F.G,S, 



The object of this paper was to give reasons for the belief that the 

 present granite of Dartmoor passed upwards into felsitic and volcanic 

 rocks, remnants of which are to be found in the Triassic conglome- 

 rate of Devon, in the detritus of the bottom lands of the moor itself, 

 on the beaches of the channel, and in ancient river-gravels and 

 pebble-beds ; to indicate the wide range of character taken by the 

 felsites of the Dartmoor district; and to point out some of the 



