740 J. W. JUDD ON THE SECONDAKY KOCJCS OF SCOTLAND. 



No. 0, 5. " Siliceous Chalk, Beinn-y-Hattan," is a Globigerina- 

 chalk, composed of Globigerina cretacea, with Teoctularia and 

 Planorbulina, and probably smaller organisms, Coccoliths, &c. 

 Compare the hard Chalk from Larne (Antrim), which is very 

 similar. Compare also the woodcuts, figs. 104-113, pp. 203-210, 

 in the English edition of Heer's i Primaeval World of Switzerland,' 

 1876, for similar rock-material and organisms. These are more 

 specially treated of in the ' Transact. Boy. Geol. Society of Ireland,' 

 November 1872, where the Foraminiferal Limestone of Antrim is 

 described. 



No. O, 6. " Calcareous Grit, U.G., Carsaig, Mull." This siliceous 

 and glauconitic grit, with calcareous cement, contains some pieces 

 of (Molluscan ?) shell and Foraminifera, such as Textularia and 

 Planorbidina. 



No. 0, 8. " Micaceous glauconitic Sandstone, U. G. Loch Aline." 

 This contains some obscure Sponge-spicules. Foraminifera, it is well 

 known, are very abundant in the Chalk, constituting a very large 

 proportion of its material. Mr. H. C. Sorby has estimated that in 

 some specimens of chalk the shells and fragments of Globigerince 

 form nearly 90 per cent, of the bulk. Coccoliths and Ehabdoliths, 

 still smaller calcareous organisms, form the finer material of the 

 Chalk. The small Entomostracan valves are sometimes in abundance ; 

 and prisms of Inoceramus-shells make up a large proportion of the 

 Chalk in some places. Polycystina have not been noticed in 

 the British Chalk. A very few Diatomaceas were recorded by 

 Ehrenberg from the Gravesend Chalk, but have not been otherwise 

 observed. Foraminifers and Coccoliths are always present. The 

 former belong to such kinds as at present live in the sea, from the 

 littoral zone down to depths of 100 fathoms or more. Some of the 

 same kinds, however, exist now at great oceanic depths ; but the 

 abyssal Globigerince of the present ocean grow much larger and 

 coarser than those found in the Chalk. 



The prepared specimens under notice from Carsaig, Beinn-y-Hat- 

 tan, and Gribun fully illustrate the theory of the pseudomorphosis of 

 the calcareous mud, or chalk- ooze, into silex, as established by Bis- 

 chofF, and adopted by Duncan, Dowker, and others. See ' Proceed. 

 Geol. Assoc' vol. iv. 1876, p. 451, &c. 



EXPLANATION OF PLATE XXXI. 



In Fig. 1 an attempt has been made to show the general relations of the 

 Volcanic rocks of the Western Isles, in illustration of the second portion of this 

 memoir, which was specially devoted to the consideration of the ancient vol- 

 canoes of the district, both those of Newer Paleozoic and those of Tertiary age. 

 In conformity with the plan adopted in the more detailed map of Mull, pub- 

 lished with the second part of the memoir, the acid volcanic rocks of Tertiary 

 age have been indicated by tints of red, the basic volcanic rocks of the same 

 period by shades of blue, the darker colour in each case representing the 

 intrusive masses, the paler the lavas, and the dotted surfaces tuffs and agglo- 

 merates. The fragments which have escaped denudation, of the great volcanoes 

 of Mull, Ardnamurchan, Mull, and Skye respectively, can readily be seen from 

 an inspection of this map, that of St. Kilda lying outside its limits. The little 



