Rev. J. F. Blake — Aggregate Deposits and Zones. 485 



conversely, the great majority of plaosphatic deposits are found to be 

 aggregates. 



If it be true that an aggregate is formed, as above explained, by 

 means of a strong current sweeping over ground whence it collects 

 the materials in its path, we have to consider what assistance we 

 obtain from it in the matter of chronology. If the fossils ai'e all of 

 one date the deposit is practically of that date also, and we may 

 have a series of aggregates overlying one another, each of a distinct 

 age, in which case their chronology is the same as that of normal 

 deposits. But when aggregates contain fossils of different dates all 

 we can say is, that the deposit is not older than the youngest of its 

 heterochthonous fossils. 



When a series of normal tranquil deposits is followed by an 

 aggregate a great change of physical conditions is indicated. The 

 work of rolling along and collecting debris is the result of 

 a disturbance of equilibrium. So soon as this is restored under 

 new conditions the aggregates are left undisturbed, and are 

 followed by a new series of tranquil deposits ; or the disturbance 

 may become chronic for a while and produce a series of aggregates. 

 Now geologists indicate these changes of condition by assigning 

 the later deposits to a new series. We may expect, therefore, 

 that aggregates will commonly be basal deposits. As a matter 

 of fact they are more often disputed deposits, but we may safely 

 say that whenever there is a dispute about the lower limit of 

 a series we are almost sure to find that an aggregate is at the 

 bottom of it. The dispute arises from the fact that the deposit 

 contains fossils of older date than its own formation, which, 

 unless its peculiar nature is observed, leads to assigning it either 

 to the age of its older fossils or to drawing a line between two 

 imjDortant series in its midst. 



It may be well asked — If these aggregates are of so great 

 a significance, have not their peculiarities been noticed long ago ? 

 I have no doubt they have been noticed by many field geologists, 

 and certainly it is many years since I first thought about them 

 what is here written ; but it is only recently, since ammonites alone 

 have been taken to indicate zones and disputes have arisen, that it 

 has seemed desirable to draw special attention to the nature of these 

 deposits by giving them a name. 



I will now point out instances of beds which I take to be 

 aggregates. The first example with which I became acquainted 

 is at Ilminster. In spite of the Geological Surve}^ map, in which 

 Inferior Oolite is marked as lying directly on Lower Lias, the late 

 Mr. Charles Moore had shown that Upper Lias fossils were obtained 

 near that town. When I was specially studying the Lias between 

 1873 and 1876 I visited the locality, and all I could find was a kind 

 of consolidated gravel in which were imbedded numerous rolled 

 specimens of Ammonites serpentinns, communis, and bifrons, which in 

 Yorkshire occupy distinct horizons, but were here all confusedly 

 mingled together. I came away with the conviction that I had 

 failed to find the true Upper Lias, and that these beds were 



