704 J. W. JTJDD ON THE SECONDAKY KOCKS OF SCOTLAND. 



exposed, specimens from more than a single band may easily be in- 

 advertently mingled together by the collector. It is worthy of note, 

 as I have pointed out in the first paper of this series, that in the 

 Eastern Highlands the Lias /3, and especially the zone of Ammonites 

 oceynotus, is remarkably well represented. 



I may here perhaps remark, while speaking of the identification 

 of the successive zones of life in the Lias, that we have in Scotland 

 some very striking examples of the manner in which a geological 

 horizon, very feebly or not at all represented in one locality, may 

 in another acquire distinctive features, important development, and 

 great thickness ; while, on the other hand, a zone, which in one 

 area may consist of thick deposits that appear to be so clearly 

 characterized as to be in capable of ever being by any possibility 

 overlooked or confounded with another, may in a different region 

 be represented by only a thin and inconstant stratum, and, more 

 than this, may lose many of its most distinctive palaeontological 

 features, the fossils which elsewhere characterized it being found 

 mingled with those of the beds above and below it. 



Thus, though I perfectly agree with Mr. Blake that in Gloucester- 

 shire and Yorkshire the Subzone of Ammonites semicostatus can 

 scarcely be separated from the Zone of A. BucJclandi, yet in Lin- 

 colnshire and the Western Highlands such a division is natural, 

 useful, and, I think I may add, indispensable. In the same way 

 the established division of the Lias y, which is so well developed in 

 Swabia and Yorkshire, might be thought in the Western Highlands 

 to be wholly unnecessary ; for scarcely any trace of it is there found. 



Nor do these remarks tend to throw the smallest discredit upon 

 the system of classifying strata into palseontological zones, when 

 that system is rightly understood and properly applied. The different 

 quantities of sediment deposited in various portions of the same 

 sea, dependent as that is primarily on the amount of subsidence 

 taking place at a particular time in certain portions of the ocean- 

 bed, will cause the same period of geological time, and the same 

 stage in the history of a fauna, to be very unequally represented by 

 thickness of deposits. The advance of our knowledge may be ex- 

 pected to result in the constant intercalation of new zones of life 

 between the old ones which had before seemed separated by distinct 

 breaks; and by this means some of the imperfections and consequent 

 anomalies of the paleeontological record will be removed. In this 

 way we shall be led to entertain more just and correct ideas as to 

 the true succession of life-forms, and have afforded to us fuller 

 materials for investigating the means by which that succession of 

 forms has been brought about. 



The most northerly point on the west coast of Scotland at which 

 traces of the Lower-Lias strata have been found is at Tinivulin 

 (Tigh-na-Fiolan), on the north side of Loch Ewe. At this place, 

 however, no Lias strata are exposed in situ ; but numerous blocks of 

 Lias limestone are found scattered upon the shore, and from these 

 I collected the following fossils : — Ostrea irregularis, Miinst. ; Ory- 

 ■phwa arcuata, Lam. : Pinna Hartmanni, Ziet. sp. ; Lima succincta, 



