Sullivan & 0'^¥.uax— On Dolomite Bed of North of Spain. 225 
XXIY. 'EOTE ON THE GEEAT DoLO&TITE BeD OF THE NoETH OF SPAIl^^, 
iisr coKJfEXioisr with the Tithomc Stage of Here Opel. By Profes- 
sors W. K. SuLLiYAN and J. P. O'Eeilly. With Plate XYIII. 
(Science). 
[Read June 12, 1871.] 
EvEF "where there is no lack of fossils, the determination of the boun- 
daries of the cretaceous and Jurassic periods is beset with difficulties. 
In the south of England the fresh-water beds of the wealden indicate, 
in the clearest manner, a break in the succession of marine deposits ; 
while in the Carpathians, Alps, the south of France, &c., the rocks of 
both periods are perhaps wholly marine. If we could recognise every link 
in each series, so as to obtain an unbroken succession of marine deposits 
from the bottom of the Jurassic to the top of the cretaceous rocks, much 
light would, no doubt, be thrown upon the great biological question 
of the changes which take place in long periods of time in the character 
of species. Much remains, however, to be done before the regular, un- 
broken succession of the rocks of these periods can be made out. The 
chief difficulty lies in what may be called the passage beds between 
the two periods. 
Between the base of the chalk — the etage valenginian, and the 
couches de Berrias — and the upper Jurassic beds characterized by 
Opelia tenuilobata, or Professor Hebert's zone of Ammonites poly- 
plocus, occur a number of such passage beds between the cretaceous 
and Jurassic periods, but more closely related to the latter than to the 
former. These beds were more or less noticed at different times by 
various Swiss and German geologists, but Herr Opel having devoted 
special attention to them, and having bestowed on them the name of 
the ''Tithonic Stage," they assumed an importance which they did 
not previously possess. 
Eocks of the Tithonic Stage are found in the Carpathians, the 
Austrian and Bavarian Alps, Switzerland, the south of France, Italy, 
and, probably, in the Sierra de Cabra, in Andalusia. Herr Zittel, who 
has described the fauna of the stage as it occurs in the Carpathian 
Mountains, recognizes two zones — the lower, or ''Zone of Terebratula 
diphya," which is everywhere in contact with the '' Zone of Ammo- 
nites tenuilobatus;" and the upper, or "Zone of Terebratula Janitor." 
The two series occur in contact at Palocsa, in the Klippenkalk region 
of the Carpathians. To the first zone belong the Stramberg beds in 
the north margin of the Carpathians ; the upper beds at Palocsa, Just 
alluded to; the Auerkalk of the Yorarlberg, the argillaceous limestones 
or Calcaire Superieur of the Porte de France, and other places in the De- 
partments of Isere, and Basses Alpes, a white non-fossiliferous lime- 
stone from the central Appenines. At Palocsa these upper beds are 
in contact with the neocomian, while the Auerkalk is in contact with 
the valenginian. The beds of the lower zone, or that of Terabratula 
K. I. A. PROC. — VOL. I., SER. II., SCIKNCE. 
