72 GEOLOGY. 



equally distinct at all stages of the period; 1 but their separation was 

 sufficient to give rise both to different conditions of sedimentation, and 

 to different conditions of life. The changes which took place during 

 the period are best understood by a study of the character and 

 distribution of the various parts of the system. 



It may be noted at the outset that the Jurassic system of Europe 

 has been studied in great detail, and that the correlation of its different 

 horizons has been carried to a degree of refinement not known in any 

 older system in Europe, and not in any system in America. About 

 thirty well defined horizons have been made out for central and western 

 Europe, and these have been found to hold over wide areas outside 

 the region where they were first recognized. The definition of these 

 horizons is based on fossils, and chiefly on the fossils of free-swimming 

 animals. The fixed forms of life, and those which are confined to 

 shallow water, ranged less widely, and their fossils do not enter into 

 the definition of the many horizons in any important way. Some of 

 the horizons which are but a few feet in thickness are traceable over 

 large areas of the continent, though not beyond the limit of a geo- 

 logical province. Thus in Great Britain, 17 distinct ammonite zones 

 have been recognized in the Lower Jura (Lias) alone, and this zonal 

 succession has been found to apply to all central and western Europe. 2 



By the definition of these provinces and by the detailed study of 

 the distribution of the various types of life within them, much has 

 become known concerning the geography of the Jurassic period beyond 

 that which is shown by the mere distribution of the Jurassic beds. 



Although the subdivision of the Jurassic system has been carried 

 to a high degree of refinement, the many zones are grouped into a 

 few principal divisions as follows: 



Germany, 2000 , -3000 / . England, 4000'-5000'. 



Upper Jura Upper Oolite 



(White Jura, Malm) (Portland Oolite) 



Middle Jura Middle Oolite 



(Brown Jura, Dogger) (Oxford Oolite) 



Lower Jura Lower Oolite 



(Black Jura, Lias) (Bath Oolite) 



Lias 



Lower Jura or Lias. — Conditions similar to those of the last stage 

 of the Triassic period affected central and western Europe during 



1 De Lapparent, op. cit., p. 1105. 



2 Geikie, op. cit., p 1136 



