Rev. J. F. Blalce — Aggregate Deposits and Zones. 487 



otherwise distinct fauna (in which case an ammonite is no guide to 

 the history of physical and faunal changes) ; or the upper bed, though 

 containing the same ammonite, is of later date, the deposit being an 

 aggregate and the fossil heterochthonous, and it marks, as Professor 

 Michalski says, the " first accident of the infra-Cretaceous boreal 

 transgression." In any case, the very distinct nature of the two 

 beds containing the same ammonite cannot be rightly neglected, 

 though the only indication of it, in one of the latest descriptions of 

 the zone, is the heterogeneity implied in the words " Marnes, sables 

 ferrugineux et glauconieux, schistes bitumineux, argiles." 



Full of these ideas, I had occasion this spring, in connection with 

 the visit of the Geologists' Association to Bridport, to consider the 

 meaning of that curious deposit in the middle of Thorncombe Cliff, 

 called by Mr. S. S. Buckman "the Junction bed," and claimed by 

 him to contain representatives of at least four separate zones. In 

 explaining the proposed excursion to the Association at their previous 

 meeting, 1 put forward a sketch of what is here written, and ventured 

 on a prophecy about this bed, which I had never seen and whose 

 lithological structure had not to my knowledge been described in 

 detail, that it would prove on examination to have the characters of 

 an aggregate. A glance at the rock sufficed to show that the 

 prophecy was correct, and I had the satisfaction of hearing some 

 members of the Association who reached it first demonstrate these 

 characters on the spot. I think, therefore, that the views I am here 

 expounding may fairly claim to stand the test of prophecy. 



About these four cases I feel pretty certain, but there are several 

 others which more or less probably belong to the same category. 

 The Ludlow bone-bed is possibly one, though its elements being 

 small, the tumultuous character is not so obvious. It is, however, 

 phosphatic, and locally marks the commencement of beds in which 

 the fossils of the underlying rocks are absent and organic remains of 

 a different type take their place. The Eh^tic bone-bed of Aust 

 Cliff is probably another. It is full of nodules of all sizes, irregularly 

 arranged, it is phosphatic, and it lies at the base of a new series. 

 The phosphate bed at the base of the Lower Greensand at Potton, 

 with its rolled bones of Jurassic reptiles and fishes, is another. 

 The nodular and phosphatic bed called the " coprolite bed " at 

 Speeton, and the " compound nodular bed " above it, occupying 

 about the horizon of the Kussian deposit, and, like it, marking, in 

 my opinion, the commencement of a new series, are further examples. 

 The mixed bed at Beer, recorded by Mr. Jukes-Browne as containing 

 the two zonal fossils Pecten asper and Ammonites 3Iantelli, may be 

 similarly explained. The " Cambridge Greensand," which has been 

 shown by the same author to contain both autochthonous and 

 heterochthonous fossils, is in like manner the base of an uncon- 

 formable series. And, finally, the Mammalian bed at the base of the 

 Eed Crag, with its huge rolled bones, its crabs from the London 

 Clay, and its nodules with enclosed ammonites, completes the list of 

 the best known of these aggregates in this country ; and they are 

 not wanting, as I can testify, on the Continent. 



