66 Geological Society .— 



galvanometer circuit ; so that the induction due to the iron 

 cylinder, apart from that due to the coil, is measured at any 

 time after the breaking of the primary circuit. The time of 

 traverse of the distance E F is determined from the trace of 

 the chronograph up to the 20j q 00 second. 



All the induction apparatus is placed at a great distance 

 from the galvanometer, so that it is in no way affected by it. 

 After the value of the deflexion of the galvanometer has been 

 obtained, the ampere-balance is used in the circuit of the coil 

 A B for determining the field due to the magnetizing coil. A 

 series of determinations of this After-Induction are now being 

 made in the Laboratory, but as the necessary reductions will 

 take some time to finish they cannot be sent with this com- 

 munication. In one experiment on a certain iron bar the 

 After-Induction took place up to 0*015 second. 



1890. 



VII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xxx. p. 281.] 



November 12, 1890.— Dr. A. Geikie, F.R.S., President, 

 in the Chair. 



HPHE following communications were read : — 



■*- 1. " On the Porphyritic Eocks of the Island of Jersey." By 



Prof. A. De Lapparent, Foreign Correspondent of the Society. 



The author had some years ago described as Permian a series of 

 porphyritic rocks, of which specimens had been sent to him from 

 Jersey. He had since been led to believe that this view of their 

 age, arrived at from what he knew of similar rocks in Prance, was 

 erroneous, and in a recent visit to the island had satisfied himself 

 that the English observers who had assigned to these rocks a much 

 higher antiquity were in the right. He now found that the igneous 

 rocks in question underlie the Rozel conglomerate, which must be 

 placed at the very base of the Silurian formations. He reserved 

 his detailed statement for a communication to the Geological Society 

 of France ; his present object being to do justice to English geolo- 

 gists, whose views he had formerly opposed. 



2. " On a new Species of Trionyoc from the Miocene of Malta, and 

 a Chelonian Scapula from the London Clay." By R. Lydekker, 

 Esq., B.A., F.G.S. 



