14 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Nov. 6, 



passing directly under the incrusting Plicatula sigillum, and there- 

 fore, evidently, produced before the growth of the shells. The stone 

 is composed of a peculiarly hard and compact siliceous limestone ; 

 and the scratches do not run in one parallel direction across the 

 fragment as in glacier-scratched stones, but are distributed in various 

 groups of parallel lines all over the surface, and especially round 

 the edges and corners. This fragment is a remarkable one ; for 

 while some of its striae are evidently ice-scratches, the majority 

 are of a very curious nature and uncertain origin. 



It may be added that occasionally a large number of fragments 

 are found associated in one spot ; six large stones, of various consti- 

 tution, were found huddled together at Waterbeach. 



Having then arrived at the conclusion that the Upper Greensand 

 erratics have been brought hither by floating ice, there next arises 

 the question as to whence they have come ; and we think, after 

 considering all sides of the case, that the facts point unmistakably 

 to Scotland and the North for an answer. Along the eastern coast 

 of Scotland, in Aberdeen, Forfar, and Fife, we have successively 

 granite, mica-schist, and basalt, Silurian, Old Bed Sandstone and 

 Carboniferous rocks ; while in Fifeshire, Edinburgh, and Berwick 

 there is a vast extension of volcanic rocks, from which the erratic 

 trap, basalt, and obsidian of the Upper Greensand might have been 

 derived. It is true that obsidian is not found in Scotland now ; 

 tachylite, however, occurs in small quantity on the Fifeshire coast ; 

 and we do not know what Volcanic rocks may have existed along 

 the extension eastwards of the Scottish coast towards Scandinavia. 

 Some of the rocks are strikingly Norwegian in character, far more 

 closely related to some of the hornblendic rocks of south Norway 

 than to any in Scotland. In deciding thus on a northern origin 

 for our erratics we are supported both by Mr. Bonney and Mr. Paley, 

 who have given some attention to the question. 



Mr. Bonney has shown that the Cambridge coprolites were ori- 

 ginally formed in the topmost beds of the Gault, and that the denu- 

 dation of these beds has given rise to the thin seam which bears the 

 name of the tipper Greensand ; and it therefore becomes a question 

 as to when the included erratics were brought down, whether 

 during the deposition of the Gault, or during its denudation and the 

 formation of the Greensand. Since w T e know that the Gault clay 

 was formed in a comparatively quiet sea, and that, after some thick- 

 ness had been deposited, change of conditions and extensive denu- 

 dation ensued, it is reasonable to attribute these occurrences in part 

 to the cold current of which we have evidence in the Cambridge 

 erratics. Some of the stones are covered by Gault coproiite, and 

 hence must have fallen into the upper beds of the Gault, during the 

 deposition of which cold conditions must have prevailed. 



The fauna, consisting partly of Gault fossils and partly of true 

 Upper Greensand forms, agrees very well with these conclusions. 

 The marine fauna is an abundant one, being, as we should expect 

 from the origin of the stratum, a mixture of deep-sea and littoral 

 forms ; but the shells are by no means large, nothing like the splendid 



