Rise and Fall of Lake Tanganyika. 379 



crystallization and liquation going on within the magma which has 

 been injected into the dyke, no such explanation is sufficient in the 

 case of the composite dykes of the second class, to which the 

 examples cited from Arran belong. 



That the association of two totally different rocks in the same 

 dyke is not accidental, the numerous and varied examples at 

 Tormore sufficiently prove. Where, as in these cases, we find that 

 there is the greatest dissimilarity between both the crystals and the 

 glassy groundmass of the two rocks, it is clear that the differentiation 

 has taken place in the magma prior to its injection into the dykes, 

 and before the work of crystallization had commenced. The physical 

 causes which have been suggested to account for such differentiation 

 in a fluid mixture of silicates are discussed. 



2. " Notes on an Intrusive Sheet of Diabase and Associated 

 Eocks at Eobin Hood, near Bassenthwaite." By J. Postlethwaite, 

 Esq., F.G.S. 



The positions of the outcrops of the igneous rock are described, 

 and a grit-band is recorded as running parallel to the diabase. The 

 diabase, and vein-stuff associated with it, have furnished antimony, 

 lead, copper, and arsenic ; and the same ores, with the exception of 

 the last two, are also found in minute grains in the grit. Analyses 

 of the grit and diabase, made by Messrs. Hellon and Brockbank, 

 are given. The igneous rock has produced slight metamorphism in 

 the surrounding rocks of the Skiddaw Slates. 



3. "On Two Dinosaurian Teeth from Aylesbury." By E. 

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



4. " On a new Plesiosaur from the "Waipara Eiver, New Zealand." 

 By Capt. E. W. Hutton, F.E.S., E.G.S. 



5. " Observations on the Affinities of the Genus Astrocoenia." By 

 Eobert E. Tomes, Esq., E.G.S. 



6. " Description of a new Genus of Madreporaria from the Sutton 

 Stone of South Wales." By Eobert E. Tomes, Esq., F.G.S. 



7. " Study of the Dykes of Hope, Idaho." By Herbert E. Wood, 

 Esq. 



In this paper a description of the geographical distribution and 

 characters of acid and basic dykes traversing slates and quartzites 

 along the northern shore of Lake Pend'Oreille, Idaho, is accompanied 

 by notes on the glaciation of the area. A brief description of the 

 microscopic features of the igneous rocks is appended. 



8. " The Eise and Fall of Lake Tanganyika." By Dr. Eobert 



The author refers to Mr. Carson's paper on the same subject in the 

 Society's Journal for 1892. He himself believes the oscillation of 

 level to be analogous to variations reported as occurring in other 



