510 R. TATE ON THE LIAS ABOUT RADSTOCK. 



Discussion. 



Prof. Duncan remarked that in the lateral extension of the Am- 

 monite-zones the forms are modified, and that species regarded in 

 one area as on different horizons, may be brought together in the 

 same zone in another district. He was glad to hear of the occurrence 

 of Ammonites angulatus at Radstock, as that species characterizes a 

 very remarkable zone in Luxembourg and elsewhere on the con- 

 tinent. 



Prof. Ramsay said that the Lower Lias consists of a regular suc- 

 cession of deposits without unconformity, reaching with us a thick- 

 ness of some 900 feet. He stated that lithologic ally there is no line 

 of demarcation between the Lower Lias and the Marlstone, and that, 

 from a palaeontological point of view, the two series cannot be divided, 

 as many of the species are common to both. If there were any 

 great divisions at all, they were between the Upper and Lower 

 Lias, and between the latter and the Oolite ; but the whole Lias 

 and Oolite really constitute one great formation, in which, from the 

 sands of the Inferior Oolite to the Lower Lias, the divisions are not 

 constant, and only apply to limited areas. Much of the fauna was 

 local. 



Rev. J. P. Blake remarked that Oppel, who was a man of great 

 and extended experience, established zones and believed in them. 

 He thought that zones might be recognized as continuous in the Lias 

 all over England, in Gloucestershire, at Lyme Regis, and in York- 

 shire. He said that there was undoubtedly a great break between the 

 Lower and Middle Lias, when the line was drawn palseontologically ; 

 and in the upper part of the Lower Lias the species diminish in 

 number, and with the Middle Lias new forms are introduced. There 

 is also a break between the Upper and Middle Lias. 



Mr. Hicks thought that these might be variations in the forms in 

 different localities. 



Prof. Ramsay said that if lines of demarcation were drawn palae- 

 ontologically and not lithologically, they might be drawn across 

 several formations that were really contemporaneous. 



Mr. Charlesworth doubted whether species could be defined, and 

 instanced cases of divergent opinions on the part of different natu- 

 ralists as to the determination of species of Mollusca. 



Mr. Koch inquired whether there might not be colonies of Ammo- 

 nites as well as of Trilobites, in accordance with Barrande's theory. 



