20 TYPE AMMONITES— IV June 



1922 



are not are, for the first, abundance of species and of specimens, good 

 preservation, delicate mouth-appendages preserved ; for the second, 

 rarity of species and of specimens, conch more or less broken, mouth- 

 appendages not preserved, body-chamber lost or considerably broken, 

 sides of camera; more or less smashed in, test showing abrasion, covered 

 with serpuhe or oysters. 



Thus the frequenc}' of Ammonite remains and their condition have 

 much information to give on pakeogcographical questions. At the 

 beginning of the Ammonitoidic Period there were no Ammonites in 

 the British area — something of Baltic Sea conditions obtained. Irruption 

 of the sea during the Caloceratan Age brought in conditions something 

 like the English Channel and North Sea — the island of Juroceltia lying 

 to the north and west. During the Clydoniceratan Age there was again 

 lack of Ammonites, for no fragment of an Ammonite from Forest Marble 

 is yet known. The brokcn-up condition of testaceous remains in that 

 deposit points to a shallow sea, much wave-action and proximity of a 

 rocky coast. Later, there is again irruption of a larger sea, with swarms 

 of Ammonites. Towards the close of the Ammonitoidic Period — in the 

 Behemothan and (iigantitan Ages — there was, o\'cr Middle and Southern 

 England, a sea \'ory favourable to the growth of large Ammonites — 

 perhaps warm and fairly deep. But this whole area must then have 

 been upraised to be drained of sea, and afterwards — in the Craspeditan 

 Age — lowered to become a large fresh-water lake — Lake Superior con- 

 ditions. Marine conditions at that time appear to have existed in 

 Yorkshire, \\'liicli would, then, ha\e to be gi\'en an outlet to the Arctic 

 Ocean. 



In those Ages, in which a west-to-east axis is mentioned as 

 making division, the conditions would have been similar to a parting 

 of the I'Inglish Channel from the North Sea by an elevation of the 

 W'ealden Axis : the English Channel conditions, with good connection 

 to open sea, prevailed to the south of the axis ; but the Cotteswold- 

 N(jrth-west England area was different from the North Sea — it was a 

 kind of ]\Iediterranean, with a strangulated outlet of Straits-of-Gibraltar 

 type, such straits lying between the Isle of Man, a north-east promontory 

 of Juroceltia, and south-west Scotland, possibly at that time a western 

 portion of the North-American Continent. 



The true problem of British Jurassic pakeogeographic reconstruction 

 lies, however, in the Estuarine Beds, which occur in Yorkshire in the 

 Ludwigian to Stepheoceratan Ages, alternating with marine beds and, 

 in the late Zigzagiceratan Age, are found there, in the east Midlands, 

 in east and in west Scotland. How long they persisted is uncertain : 

 they might have been followed by marine strata of, say, Tulitan date 

 which have been wholly destroyed in some of the areas — there is some 

 exidence for this supposition in the east Midlands. But at an 3^ rate 

 marine conditions were again general by Macrocephalitan date. But 

 the problem is to find the water for the rivers to make these estuaries, 

 for they cannot be connected up to one river only. Local British supplies 

 would be insufficient ; but if a river be brought from Iceland way to 

 make its estuary in Western Scotland, and the drainage of Scandinavia 

 be invoked for the liastern Midlands, there still remain problems of 

 liastern Scotland and Yorkshire. The ri\-ers evolved should approximate 

 to som(> i)resent-day geographical types, and the estuaries must be 

 capable of conversion into seas according to the demands of autoch- 

 thonous or of drifted Ammonites for the respective areas and times 

 concerned. 



