486 Rev. J. F. Blahe — Aggregate Deposits and Zones. 



disturbed and perhaps redeposited, I now know that, though 

 I certainly failed to find the Fish-bed with its truly autochthonous 

 fossils, it is these very rubble-beds which have been taken to 

 indicate the several ammonite zones. I should now call them 

 an aggregate, and assign the date of its formation to the com- 

 mencement of the Inferior Oolite period. 



Another example is the Gloucestershire Cephalopoda-bed as seen 

 at Frocester Hill. The underlying sands appear to be normal 

 deposits, and they contain Liassic ammonites, but the four feet 

 or more which constitute the capping contain numerous species 

 of ammonites and other fossils all confusedly mixed together, so 

 that no definite subdivision of the bed is possible. The mingling 

 in this bed of Liassic ammonites with others of later date has led to 

 much dispute over its correlation. Its character suggests that it is 

 the commencement of a new series, and the other fossils strongly 

 confirm this view, for many are found in the Inferior Oolite and not 

 in the true Lias, the most noticeable perhaps being PJioladomya 

 fidicula. It is not a case of passage beds at all, but of a marked 

 break, with a mere mechanical mingling of fossils of different 

 horizons. If its peculiarity as an aggregate is overlooked, we 

 might unite a part of it with the sands below as forming 

 a single zone — that of A. sfriatulus — as has actually been done, 

 thus comprising within the limits of a zone the junction (according 

 to what appears to me to be the truth) of two formations and 

 certainly of two deposits of an entirely different character, and 

 containing to a large extent a different fauna. 



It was a similar phenomenon, observed in Eussia last year on the 

 occasion of the visit of the International Geological Congress, that 

 persuaded me of the great importance of observing the character of 

 these aggregates, so as to avoid the confusion that otherwise arises. 

 In the upper part of the " Jurassic " rocks of that country, there 

 occurs a well-marked ammonite whose transverse ribs seem collected 

 into bundles, and hence it is called Ammonites (or Virgatites) virgatus. 

 This species is taken as a zonal ammonite, and all deposits which 

 contain it are grouped together as a single zone by those who are 

 guided by ammonites alone. In the greater number and most 

 accessible of its localities it occurs as a heterochthonous fossil, in 

 spite of which it has been taken to characterize the epoch of the 

 deposit. In one locality, however, it is found in underlying beds as 

 a truly autochthonous fossil. It there occurs in thin bituminous 

 shales, exactly of the character of our own Upper Kimmeridge clay, 

 and is accompanied by the characteristic fossils, Lingula ovalis and 

 Discina latissima. There can be no doubt, therefore, that its true 

 home is the Upper Kimmeridge. In the same locality it occurs also 

 in the overlying aggregate deposit as heterochthonous fragments, 

 and is accompanied by a different group of other fossils. We have 

 here, then, the following alternative : either a single ammonite may 

 survive the change from a very tranquil deposit, following on a long 

 series of similar deposits to a tumultuous one of an entirely different 

 material, followed by other tumultuous deposits, and containing an 



