492 Crosby and Ballard — Distribution and Age of 



oxidized zone and are due to the very local cementation of the 

 till by carbonate of lime, the most probable source of this 

 cement being fossil shells which have vanished in this trans- 

 formation. These masses of recent conglomerate or pebbly 

 claystones range from a small fraction of an inch to several 

 inches in diameter ; are usually distinctly rounded in outline ; 

 and may occur loose in the till or attached to larger stones and 

 bowlders. Although observed in several sections, they are 

 especially noticeable in the buff till of Telegraph Hill ; and 

 one example from this section shows where broken the impres- 

 sion of a shell fragment, probably Scapharca transversa, with 

 a minute portion of the original shell still remaining. 



In part to the interstitial deposition of carbonate of lime 

 from dissolved shells, but chiefly to the peroxidation of the 

 iron oxide in the till, is due the setting or imperfect lithifaction 

 of the buff till which causes the upper portions of the cliffs 

 to present, usually, very precipitous or even vertical and over- 

 hanging profiles. That the buff till and the blue till are 

 really contrasted in this respect is apparent from the fact that, 

 no matter how high or how fresh the sea-cliff may be, the 

 talus starts, as a rule, from near the junction of the two kinds 

 of till, and the blue till can be seen in situ only where the 

 slope is gullied. 



The segregations of carbonate of lime are also found on 

 the shells themselves, below the zone of most perfect oxida- 

 tion of the till ; the localities where this relation has been 

 specially noted being Moon Island, the east and west ends of 

 Peddock's Island, Deer Island, Point Allerton and Strawberry 

 Hill. They occur almost exclusively on the larger and more 

 massive shells, such as Venus and Cyclocardia ; and are attached, 

 as a rule, to the inner surfaces or the less salient portions of 

 the shells, often filling the recesses of the hinge and the per- 

 forations made by the boring sponge — Cliona sulphurea. 

 Occasionally, however, the shell fragments are almost entirely 

 enveloped, forming the nuclei of concretions. In some cases, 

 the segregations attached to the shells are protuberant and 

 present the rounded outlines of normal concretions ; but more 

 commonly they have a worn or fragmentary character, as if 

 they had suffered abrasion subsequent to segregation. These 

 same shells frequently show glacial strias ; but in no instance, 

 so far as we have observed, do the strise cross the cemented 

 material, although, on account of the rough and gritty char- 

 acter of the latter, one might well hesitate to speak positively 

 on this point, but for the fact that prominent scratches are in 

 some cases partly filled and covered by this natural concrete. 

 This is an important point, since, in connection with the con- 

 cretionary form of the segregations, it affords a conclusive 



