﻿456 Henry JS. Soicorth — Geology of iJie Isle of Man. 



VIII. — Geology of the Isle of Man. 



By Henry H. Howokth, Esq. 



{Concluded from page 413.) 



IN my former note I adduced the reasons whicli make me confident 

 that the conglomerates of the south of the Isle of Man, which 

 have been hitherto classed as Devonian, are not Devonian, but are 

 later than the Carboniferous Period. I will now turn to the problem 

 of what they really are. It is a well-known fact that in the Isle of 

 Man the Secondary and Tertiary rocks are wholly absent. This is 

 the view of all those who have examined the island, and is in fact 

 perfectly palpable. The series of older rocks terminates with the 

 upper layers of the Carboniferous Limestone; above this there 

 are nothing but deposits of Quaternary Age. The red con- 

 glomerate to which I have referred contains no trace of any Second- 

 ary or Tertiary fossils, nor is there the smallest ground for believing 

 that it belongs to either of those series. Does it then belong to the 

 Quaternary beds ? This conclusion is inevitable, and, as we shall 

 see, is an exceedingly interesting one. 



The deposits of the Pleistocene age are developed in the Isle of 

 Man on a very lai"ge scale, and the series may be there studied in 

 detail, and especially the Boulder-claj'^ formation. One of the 

 peculiarities of the Boulder- clay in the Isle of Man is the large 

 proportion of local or insular materials that have built it up. 

 This has been noticed by Mr. Gumming (op. cit. page 113). 



Here let me make a small digression. If, as is almost certain, 

 the matrix in which the boulders are imbedded was formed out of 

 the subjacent rocks, ground down and pulverized by moving ice, 

 it is quite clear that what will be clay in one situation, where the 

 materials for making clay exist, will be an entirely different sub- 

 stance elsewhere. That the same sheet of shore-ice, which in scrap- 

 ing over the upturned edges of Silurian schists makes excellent clay, 

 will, within a few yards of the same place, and perfectly contem- 

 poraneously, be making a bed of a pasty texture out of the Mountain 

 Limestone immediately adjoining, while if it was ploughing down 

 the surface of Secondary and Tertiary rocks, the resulting product 

 would again be different. I don't know (and claim the privilege of 

 a mere student in suggesting) but it may be that those Avho have 

 mapped out the Quaternary Deposits of Europe have not sufficiently 

 attended to this crucial fact, namely, that they have looked for Boulder- 

 clay where no clay could possibly be found, and that they have placed 

 on a different horizon some really contemporaneous bed made of dif- 

 ferent materials. 



In the Isle of Man, at all events, we must take this fact into con- 

 sideration, and we have our lesson very palpably laid down for us. 

 Where the basset edges of the schists are exposed, we find the ad- 

 joining drift deposits to be almost pure clay. Where these beds 

 change colour, so do the superincumbent clays. Where we near the 

 horizontal beds of limestone deeply scored in various directions, we 

 find more and more lime in the clay, which becomes a veritable tilth, 



