part 4] juEASsic cheoxolouy. 437 



(ii) a thin ferruginous layer (N) with Harpoceraioides spp. below 

 the limestones of t\\e falciferum layer at Thorncombe Beacon ; 

 and (iii) a marly laj^er (NJ at the top of the serrata bed, also at 

 Thorncombe Beacon. 



The blue sandy ' Upper Lias ' claj'-s everywhere rest non- 

 sequentially upon the limestones of the Junction-Bed with a 

 remarkably sharp line of division ; and, owing to the impersistence 

 of the several layers of the Junction-Bed, the clays may rest upon 

 any layer from tlie striatiihim to the falciferum zone. Usually, 

 the basal 6 to 12 inches of the clays become more sand}^ and 

 ferruginous tlian the main mass above, and this lower portion 

 contains a good many coarse-ribbed Dumortierias in the condition 

 of friable casts — mostly too rotten to be preserved. A long 

 slender Belemnites sp. occurs, but is uncommon. 



The lithic character of the striatnlum laj^er varies much. In 

 parts it is a hard, rubbly, greyish-yellow limestone ; in other 

 sections it is a soft earthy marl, with harder lumps and man}'- 

 limonitic nodules and rolled fossils in a perished condition. At 

 the base is a very thin, impersistent seam of dense laminated 

 limestone showing an obscure mammillated structure, suggestive of 

 * Gotham Marble.' Whenever the top of the striatulum layer cans 

 be examined in situ, it supplies plain evidence of long-continued 

 quiet erosion, prior to the deposition of the ' Upper Liassic ' clays,, 

 being planed off: perfectly level and displaying ammonites in sec- 

 tion — the lower or embedded side well preserved, the upper- 

 removed b}^ the erosion. From a fallen block below Down Clift' I 

 collected a large alticarinate Havgia, 5322 ^ ^Esericeras aff. eseri 

 (Oppel), more umbilicate], almost as accurately cut through as if 

 sliced on a lapidary's wdieel : tlie erosion has removed the shell 

 down to within a few millimetres of the thin high keel almost 

 equally around the greater portion of the circumference of the 

 shell. The striatulum l^eiyev reposes upon the smoothed top of 

 the hifrons layer with a well-marked plane of division, but 

 in perfect conformity. There is no obvious indication of the 

 extensive non-sequence known to exist — a good illustration of the 

 kind of deceptive conformity that has led to so manj^ under- 

 estimates of the importance and real time-value of attenuated 

 deposits. 



The hifrons layer is a hard tough limestone, usually mottled 

 pinkish-3''ellow and red. Sometimes the stone is Avholly fine- 

 grained and massive ; generally it abounds in pebbles of a similar 

 rock, together with derived Hildoceras hifrons J^ The derived 

 ammonites are often reduced to mere limonitic pebbles showing 

 traces of the sutures, etc. Unworn examples of H. hifrons occur 

 among the redeposited specimens. The upper 2 or 3 inches of the 

 hifrons layer is a greyish-yellow rubbly limestone, abounding in 

 small specimens of TI. aff. hifrons and DactyJioceras cf . commune ; 



^ These numerals refer to my register of specimens. 

 - Trivial names of species are used sen>iU Into. 



