R. Etheridge, Jim. — Glacial Deposits of Lewis. 555 



Boulder-clays near the Butt, witli the view of determining, if possible, 

 the succession of physical changes that ensued after the deposition 

 of the unfossiliferous bottom till which covers so wide an area in the 

 Long Island. I have given elsewhere (" Great Ice Age," 2nd edit. 

 chap, xvi.) a detailed account of the evidence, and only state here in 

 a few words the genera conclusions arrived at. They are these : 

 1. Period of intense glaciation : Lewis overflowed by ice coming 

 from the mainland : formation of unfossiliferous bottom till. 2. Re- 

 cession of the ice-sheet : ice vacates the Minch, which becomes 

 tenanted by northern mollusca. 3. Eeturn of ice-sheet: ice over- 

 flows north part of Lewis : formation of lower shelly till or 

 Boulder-clay. 4. Eecession of the ice-sheet : partial submergence 

 of the land : arctic and boreal shells live in the Minch : formation of 

 stratified deposits between the tills. 5. Return of ice-sheet : ice over- 

 flows north part of Lewis : formation of upper shelly till or Boulder- 

 clay. 6. Final retreat of ice, followed by partial submergence."] 



Before closing these remarks I wish to notice certain peculiar 

 burrows penetrating a piece of shell, apparently that of Cyi^rma 

 Islandica from Triagh Chrois. The markings are of two kinds. 

 The first are fine straight grooves intersecting one another at various 



angles, the second are formed of a 

 minute perforation or foramen with 

 a fan -shaped series of ramifying 

 tubuli, always on one side of the 

 foramen (Figs. 1 & 2). The speci- 

 men was submitted to the Rev. 

 A. M. Norman, who kindly in- 

 Fio-. 1. * Fio-. 2. ** formed me that both kinds of per- 



forations are probably due to fungi. 

 He thinks that the fan-shaped perforations are caused by a fungus 

 in a state of fructification of similar form to Stijsanus caput-meduscB.^ 

 Professors Kollicker and Wedl consider that all the tubuli described 

 by Dr. Carpenter in the shells of Bivalves are caused by para- 

 sites ; the former ascnbes them to Unicellular Fungi, the latter 

 to Confervse,^ An interesting fact in connexion with these fan- 

 shaped perforations is that Mr. C. W. Peach has in his cabinet 

 several pieces of Cyprina Islandica, and other shells, from the 

 Caithness Boulder-clay, covered with them. The above figure, 

 from a drawing by Mr. B. N. Peach, represents the perforations 

 as seen on the Hebridean specimen from the laminated beds at 

 Traigh Chrois. I have in conclusion to express my thanks to 

 Mr. H. B. Brady for determining the Foraminifera, to Dr. Gwyn 

 Jefi'reys for checking my determinations of the Mollusca, to the Rev. 

 A. M. Norman for notes on the above figure, and to Mr. J. Bennie for 

 assistance in cleaning andsorting the fossils. 



1 See Carpenter, The Microscope, 5tli ed. 1875, p. 389. 



2 Kollicker, "On the Frequent Occurrence of Vegetable Parasites in the Hard 

 Stnictui-es of Animals," Proc. Eoyal Soc. vol. x. p. 95. 



* Series of fan-sliaped ramifying tubuli seen in shell of Cyprina Islandica. 



* * A single fan-shaped mass very highly magnified. 



