gumming on the isle of man. 335 



Part II. 

 On the Tertiary Formations of the Island, 



[Read February 4th, 1846.] 



I have already stated the nature and extent of the difficulty that 

 exists in the way of obtaining a correct geological history of the 

 older rocks in the Isle of Man ; and I have alluded to the fact that 

 the beds in general, especially those which are most instructive, 

 and which appear on the southern side of the island, are covered 

 up by tertiary sands and clays. I propose now to describe these de- 

 posits. 



Although the existence of a large marine formation in the north 

 of the island, containing shells which have been referred to the 

 post-pliocene or pleistocene age, has already been made known 

 through the labours of Professor E. Forbes and Mr. Strickland*, 

 the occurrence of similar beds in other parts of the island appears 

 hitherto to have been overlooked. It is however only by a close 

 connexion and comparison of the phaenomena presented at these 

 other localities that the real character of the deposit and its relation 

 to the older rocks can be determined, the base of the formation not 

 being exposed in the north ; whilst on the other hand, the superior 

 thickness of the strata there developed and the fossils contained, 

 supply evidence rarely to be obtained in the other district. 



The deposits which I have now to describe may be classed under the 

 four heads : — 1. boulder clay and erratic blocks ; 2. diluvium ; 3. drift 

 gravel, and 4. alluvium ; and I propose to describe in detail the posi- 

 tion and relations as well as the nature and contents of each of these. 



1. Boulder Clay. — This deposit consists for the most part of a mass 

 of unstratified, or very irregularly stratified loam and sand, including 

 boulders of local and foreign rocks, of which the former (those from 

 the neighbourhood) are generally the most remarkable in point of size, 

 and are not much rounded, although scratched and polished on their 

 flat surfaces. These boulders are frequently pushed over one upon 

 another, and this is especially the case with the fragments of lime- 

 stone in the south. The lower part of the deposit is the most loamy, 

 and in the upper part the sands are scattered in irregular patches, 

 and often exhibit a waved appearance. 



Throughout the whole, fragments of shells are diffused sparingly 

 and in an extremely broken and comminuted state; they are chiefly 

 abundant in the sands, but are generally too friable to be removed 

 without injury. Those that have been examined seem to belong 

 to tiie pleistocene or newest pliocene period of Mr. Lyell ; but 

 in the lower portion of the deposit some species have been found 

 identical with those hitherto considered as confined to the Crag; so 

 that the date of the formation may perhaps extend so far back as 

 the older pliocene t« 



There is yet greater difficulty in determining the superior limit 



* Proc. Gcol. Soc. vol. iv. p. 8. f See List of Fossils, p. 340, 



'^2 



