J. A. Birds — On the Isle of Man. 81 



upon the geology of the island, besides some Memoirs published in 

 the infancy of the science, or a few brief notices of special points 

 since. 



Mr. Cumming, who for many years was Vice-Principal of King 

 William's College, Castletown, appears to have studied the geology 

 of the little country very thoroughly, and has left an excellent 

 account of it, illustrated with several maps and sections. Pro- 

 bably the Geological Surveyors, when they come to explore the 

 island, will find little to correct or add to in the portion of Mr. 

 Cumming's works which relates to the older rocks — unless it be 

 to determine positively the age of the Silurian schists. 



With regard, however, to the later, or Post-pliocene accumula- 

 tions, the progress made in this portion of geology since the publi- 

 cation of Mr. Cumming's "History" in 1848, will, I think, 

 necessitate very considerable changes. 



Mr. Cumming divided these formations into two classes : 1. 

 Boulder-clay ; 2. Drift-sand and gravel ; and he seems to have 

 regarded the Boulder-clay as all of the same age. This I believe 

 to be a fundamental error, which throws into confusion the whole 

 account of the order of these deposits. 



It is hardly necessary to remind readers of the Geological 

 Magazine of the three or four great periods in the hypothetical 

 history of the last geological age of the British Isles, and of the 

 whole of Northern Europe and America, as it is summed up by Sir 

 C. Lyell in the last editions of his ' Principles ' and ' Elements,' and 

 in the ' Antiquity of Man,' viz. : — 



1st. A continental period, when the land was much higher than 

 at present, and all, except perhaps the summits of the highest moun- 

 tains, was covered with a thick sheet of ice. 



2ndly. A period of gradual submergence, in the later part of 

 which icefloes and icebergs drifted to and among these islands, and 

 their highest portions formed an archipelago in the North Sea. 



3rdly. A period of emergence ending in a second continental con- 

 dition, when, however, the land was probably not so high as in the 

 first period, though glaciers occupied the higher valleys. 



8. "On the Geology of the Isle of Man." By the Eev. J. G. Cumming. 



Part I. Palaeozoic Bocks. Part II. Tertiary Formations, with Plates 

 xiv.-xvii. Quart. Journ. Geol. Soc. vol. ii. 1846. 



9. " On the Geology of the Calf of Man," by the same. Quart. Journ. Geol. 



Soc. vol. iii. 1847. 



10. " The Isle of Man : its History," etc., by the same. London, Van Voorst, 1848. 



11. "On the Superior Limits of the Glacial Deposits in the Isle of Man," by 



the same. Quart. Journ. Geol. Soc. vol. x. 1854. 



12. "A Guide to the Isle of Man," by the same. London, Stanford, 1861. 



13. " On certain Tracks in the Manx Slates." By Thos. Grindley, Esq. Geol. 



Mag. Vol. II. p. 542. 1865. 



14. " On the Geology of the Lake District and the Lower Silurian Bocks of the 



Isle of Man." By Professors Harkness and Nicholson. Quart. Journ. 

 Geol. Soc. vol. xxii. 1866. 



15. "Practical Guide to the Isle of Man." By H. J. Jenkinson. London, 



E. Stanford, 1874. Contains Chapters on the Mineralogy and Geology 

 of the Island. 



DECADE II. — VOL. II.— NO. II. 6 



