108 MESSES. A. J. JFKES-BKOWNE AND W. HILL : [May 1 896, 



How much of the Lower Chalk should be regarded as belonging 

 to the zone of Ammonites varians is doubtful, but it probably 

 includes the sandy chalk with phosphates, and this would make a 

 thickness of 53 feet. 



It is especially noteworthy that there can be no doubt where the 

 line between Chalk and Greensand should be drawn in this section. 

 Here everyone has taken the same horizon, for there is no passage 

 as in the Isle of Wight, but an abrupt change or break at the base 

 of the Chloritic Marl, which rests on an irregular surface of the 

 underlying sandstone as if a certain amount of current-erosion had 

 taken place before its deposition. 



The nodular sandstone is clearly the equivalent of the beds which 

 contain similar concretions and the same fossils in the Isle of Wight, 

 but the chert-beds are absent ; as, however, they are only 13 feet 

 thick at Compton Bay according to Mr. Strahan, it is not surprising 

 to find them absent here. They are in fact a local and variable set 

 of beds, and are absent over a large part of Dorset, though where 

 present they always come in at the same horizon, and, as we shall 

 presently see, they set in again a little farther west. 



At Ballard Hole Pecten asper has only been found in the nodular 

 sandstone, but as the sandy material of this bed passes down into 

 the greensand below it is impossible to say how much should be 

 included in this zone. There is in fact a complete passage down 

 into sand that contains Ammonites rostratus. 



As in the Isle of Weight, different observers have drawn the line 

 between Greensand and Gault at different horizons. Mr. Fordham 

 takes the Upper Greensand down to the lowest stone bed and thus 

 gives a thickness of 71 feet to this division. Mr. Strahan draws 

 the line higher up, assigning only 45 feet to the Greensand and 

 110 to the Gault; Neither have attempted a division into zones, and 

 we do not yet know how much of this thickness should be assigned 

 to the zone of Ammonites rostratus, and how much to those of 

 A. lautus and A. interruptus ; it is not probable, however, that 

 the division into zones would coincide with either of the lines taken 

 to separate Gault from Greensand. 



The next fairly complete and accessible section is at Lulworth. 

 The Lower Chalk was measured here by one of us in 1892, and 

 found to be only 38 feet thick, a remarkable diminution of thickness. 

 Dr. Barrois gave a detailed account of the Greensand portion in 

 1876, 1 and Mr. Meyer has kindly supplied us with a note on the 

 beds at the junction of the Chalk and Greensand. Combining these 

 sources of information, we have the following sequence : — 



1 ' Eecherches sur le Terr. Crct. Superieur, etc.,' p. 89. 



