

'I I IK .JURASSIC SYSTEM 409 



marls, and shales, with occasional coal-seams, and this 

 ads through Hungary to the Carpathian Mountains. It is 

 evident, therefore, that the sea shallowed rapidly to the east as well 

 as to the north. 



Beyond this in Russia no representative of the Lias is 

 known, the lowest beds of the Jurassic System in Central Russia 

 belonging to the Cornbrash or the Callovian. Liassic deposits, 

 however, are found again to the south-east in Servia, Croatia, and 

 Dalmatia, and again in the Caucasus, while littoral beds with 

 plant remains (possibly of Liassic age) have been found at Isjoum 

 on the Donetz in South Russia. 



II. MIDDLE JURASSIC SERIES 



A. STAGES AND ZONES 



This series has always been divided into two stages ; a lower, 

 known as the Inferior Oolite Group in England and the Bajocian 

 in France, and an xipper, best called the Bath Oolite Group and in 

 France the Bathonian. The precise limits of these stages have, 

 however, been matters of dispute, and are not yet settled. As 

 stated on p. 387, the most convenient horizon at which to draw 

 the line between the Lower and the Middle Series seems to 

 be at the base of the opalinum zone. This places the base of the 

 Inferior Oolite at an horizon which can easily be followed all 

 across England, and it is at this horizon also that the base of the 

 series is taken on the International Geological map of Europe. 



With respect to the upper limit of the Bajocian or Inferior 

 Oolite, there is a growing prevalence of opinion that it must be 

 placed at a lower horizon than formerly, and that the zone of 

 Parkinsonia Parkinsoni should be included in the Bathonian ; for 

 the fauna of this zone is more closely related to that of the beds 

 above it than to that below, and in England there is a marked 

 stratigraphical break at its base. Again, there can be no doubt 

 that whatever line is taken as the plane of separation between the two 

 stages, this line should be drawn at the same horizon both in England 

 and France, so that the stages should be completely correlative. 



During the last ten years much detailed zonal work has been 

 done both in England and France, and a large number of zones 

 and sub-zones (or hemerae) have been proposed. For those who 

 make a special study of this series such work is of much value, but 

 for the general student it will suffice to indicate the broader zones 

 which can be traced over wide areas in England, France, and 

 Germany. These are given in the following table : 



