TYPE AMMONITES— IV June 



this country, that the extension of some of them to the Continent, 

 if it does not fail ahogether, is but partial, and that their synchronization 

 with Macrocephalite deposits further afield is quite doubtful. Strict 

 faunal analyses will, it is to be expected, show that no area possesses 

 the full sequence of Macrocephalitan deposits, but that what have been 

 preserved are only odd fragments of different dates — an assortment 

 varying from place to place. 



Analyses of the deposits of the Jurassic (Ammonitoidic) Period 

 may be expected to show similar results for the strata of all the different 

 Ages — that is to say, that possibly no locality possesses a complete 

 sequence, even for quite a short duration of time ; that what has been 

 lost from any one place is considerably more than what remains ; that 

 the tale of strata has to be built up from series of locally-preserved 

 fragments, which are sequentially very incomplete ; and that, therefore, 

 the task of correlating the strata is particularly difficult — all the more so 

 because of the phenomenon of faunal repetition. It follows, too, that 

 if there are so many gaps in the true tale of strata of any one area, the 

 time occupied in the deposition of the strata of the Jurassic Period is 

 far greater than what may be estimated from an area of thickest deposits. 

 For if strata supposed to be synchronous, supposed to be the deposit 

 made during one hemera, are found to be really fragments of sequential 

 deposits made during several hemerae, and can, by their overlapping 

 in time, here and there — A followed by B at one place, B by C, 

 A missing, at another, C by D at a third locality — be fitted, after the 

 manner of a puzzle, into a sequence, the great increase in the number 

 of hemer^e and of Ages follows logically as a necessity, rightly to express 

 the chronological phenomena. Then, to obtain the time occupied in 

 deposition, the maximum thicknesses of deposits for each hemera must 

 be added : A may be two inches thick at one place, and loo feet at 

 another, B at these places may be just the reverse — the full thickness 

 of deposit made during A, B, on which a time-estimate must be based, 

 is, therefore, 200 feet, not 100 feet and two inches. 



Even when all these data have been collected, allowance would have 

 to be made for possible losses. The deposits of a hemera which were 

 once laid down in great thickness may have been so denuded that only 

 a few inches are left, or they may have been destroyed without leaving 

 any trace. A little more denudation would have utterly destroyed all 

 trace of the deposit made during the hemera of Zigzagiceras poUuhrum — 

 would have effaced all trace of it from the localities in Dorset-Somerset 

 as effectually as it has done from all other places in England, on the 

 Continent, and, as far as is known, from the rest of the world. It is 

 impossible to imagine that what so nearly happened in this case has 

 not actually hai)pened in others — perhaps in many cases. Gaps in 

 the faunal record may give some clue as to where such losses have 

 occurred. 



One further point : by the aid of a greatly-extended hemeral system 

 of chronology, but only by such a system, it should be possible to map 

 the lines of denuded areas, hemera by hemera. Then it will be seen 

 whether these lines coincide, or whether, as is likely, they have been 

 propagated in a wave-like manner more or less parallel to certain known 

 lines of weakness. The possible importance of such research, with the 

 knowledge it should give for economic questions is obvious. 



The main lines of weakness in England are, north to south, two — 

 the Malvernian axis and the Pennine ; west to east, two, the Mendip 

 axis and the North Devon axis. South of this, and roughly parallel, 



