416 Geological Society. 
cemented by silica, and sometimes by calcite. The rock in contact. 
with the dykes sometimes contains quartz in isolated bipyramidal 
crystals and granular aggregates. The silica is present in the lime- 
stone in two ‘forms, which have had an entirely different origin. 
Reference is given to examples of sandstone-dykes hitherto 
described, and then the origin of the quartzite-dykes at this locality 
is discussed. An important bed of sandstone was tound by sinking 
for a well at Marston-Common Farm; and the same bed is found 
also in a quarry about 800 feet south of the farm. The microscopic 
aspect of the rock is precisely similar to that of the dykes. It is 
at a period later than the Keuper that the silica which cemented 
the sandstone of the dykes and of the Common Farm appears to 
have been introduced. 
2. ‘Phenomena bearing upon the Age of the Lake‘of Geneva.’ 
By C. 8. DuRiche Preller, MA), Pa. Dy ., A-M.LCJE., MTs 
F.R.S.E., F.G.S. 
Following up his investigations concerning the age of the 
principal Alpine lake-basins, the author has, during a recent 
prolonged stay on the Lake of Geneva, examined the low-level 
gravel-beds and other alluvia in the Rhone Valley, from Geneva 
to the Jura-bar near Fort de l’Ecluse, as well as the high-level 
gravel-beds of La Céte above Rolle and of the Jorat district above 
Lausanne, and, further, the rock-formations on both sides of the 
lake, in view of evidence of flexures as the primary cause of 
the formation of the present deep lake-basin. 
After describing the phenomena around the Lake of Geneva, and 
comparing them with those around the Lake of Zurich, he is led to 
the following conclusions :— 
(1) The low-level gravel-beds of the Rhone Valley near Gelgetal 
are, like the deep-level gravel-beds of the Limmat Valley 
near Zurich, fluviatile deposits of the second interglacial 
period, and were formed before the present deep lake-basin 
came into existence. 
(2) The high-level gravel-beds of La Cate above Rolle and of the 
Jorat district above Lausanne are, like the corresponding 
deposits of the Uetliberg near Zurich, and of the Dombes and 
of Lyons, true Deckenschotter. Hence the term ‘ alluvion 
ancienne’ should, in its proper acceptation, only apply to 
the high-level deposits. 
(3) The formation of the present deep lake-basin of Geneva was, 
like that of Zurich, primarily due to the lowering of the 
valley-floor by flexures of the Molasse and its contact-zones, 
posterior to the maximum glaciation, as evidenced more 
especially by the reverse dip of the old erosion- terraces. 
The author holds that the concord of evidence in the two cases 
strengthens the conclusion, already arrived at by analogy in his 
previous paper, that the Lake of Geneva, together with the other 
principal zonal lakes between the Alps and the Jura, was formed 
under similar conditions and at the same time as the Lake of 
Zurich, that is, towards the close of the Glacial Period; indeed, the 
phenomena in support of that view are, in the case of the Lake 
of Geneva, on a grander scale, more striking, and, if anything, 
more conclusive. 
