LIAS. 257 



has long been famous for its snake-stones [Ammoitifcs), but so great has been the 

 demand for these, that suppHes have been sent there from other localities such as 

 Lyme Regis — a fact which collectors should bear in mind, as few fossils are of 

 any scientific value unless their locality be definitely known. 



Those who have studied and written about the Lias have un- 

 fortunately not adopted a uniform system in dividing the beds into 

 zones. This arises in part from the fact that in different areas the 

 organic remains are variously distributed, and subdivisions may be 

 made in one tract that cannot be distinguished in another. More- 

 over the zone marked by a certain Ammonite in one tract may be 

 marked by another (different or representative) species elsewhere. 



The Lias was divided into Upper, Middle, and Lower divisions 

 by John Phillips in 1829.1- 



The Table given on p. 258 will be useful in showing the 

 groupings adopted by different geologists. 



It is admitted that the line between the Middle and Lower Lias 

 is geologically a vague and indefinite one, for being taken in a 

 series of clays, there is seldom any physical feature either in the 

 form of the ground or in the strata themselves to mark the 

 boundary. Hence, whether the line be taken above or below 

 the zone of Ammonites capriconius or that of A. Henhyi, it can only 

 be determined by palasontological evidence. This evidence in 

 itself suggests no break, and no reason for drawing a line ; thus 

 opinions differ on the question where the Lower Lias ends and the 

 Middle Lias begins. Were we to adopt divisions based on 

 lithological characters, the Lower Lias limestones would form a 

 lower group ; the Lower and Middle Lias clays, a middle group ; 

 and the Middle Lias sands and Marlstone (Rock-bed) an upper 

 group. The Middle Lias sands are however not developed in 

 Lincolnshire as they are in Dorsetshire; moreover Ammonites 

 margaritatus is prevalent in the clays immediately underlying 

 these sands in the south of England, and it occurs also in the 

 rock-bed of that area together with A. spinatus. Hence neither 

 palaeontologically nor lithologically can satisfactory artificial 

 divisions of the Lias be made. Whichever boundary-line is 

 advocated will be open to some local objections ; but as the 

 Geological Survey has adopted the line taken by Prof Judd, 

 and as it has been found a fairly good divisional-line in York- 

 shire, Lincolnshire, and Northamptonshire, it is adopted in this 

 work. The Table given on p. 259 shows the geographical distri- 

 bution of the Ammonite-zones in this country. 



Lower Lias. 



The Lower Lias consists in its lower portion of blue and brown 

 argillaceous limestones ('Blue Lias'), with partings of clay or 

 shale, often bituminous; in the higher portion the clay ('Blue 



^ Geol. Yorkshire, part i. 



17 



