THE CRETACEOUS ROCKS OF RRlTALN. 
124 
CHAPTER X. 
THE ARENACEOUS BEDS (ZONE OF AMMONITES 
MANTELLI) IN DEVON. 
General Description. 
The distance from the outlier of Lower Chalk at Membury to the 
Bindon Cliffs, near Axmouth, is a little over 8 miles, and from 
Membury westward to Wilmington is about 44 miles, yet within 
these intervals a remarkable stratigraphical change takes place. 
The Lower Chalk (as chalk) entirely disappears, and in its place 
there is a variable thickness of calcareous sandstone, overlain by a 
bed of compact quartziferous limestone. It is unfortunate that 
no outlier has been found within the intermediate spaces to help 
us in understanding the precise manner in which the chalky 
facies disappeared or passed into the sandy facies. 
The arenaceous beds are exposed in the coast sections from 
Lyme Regis nearly as far as Sidmouth, hut inland the only good 
exposures are in the outlier at Wilmington, which was described 
in 1898.* * 
These beds appear to represent a part of the Chalk Marl, but as 
they are neither chalk nor marl, and as Am.[Acanth.] Mantelli is one 
of the most common and characteristic fossils in them, it is better 
to speak of them as the zone of Am. Mantelli. This appellation is 
the more appropriate inasmuch as the lower part of the Cenomanian 
in the west of France has been described by M. Bizet as the “ craie 
glauconieuse a Am. Mantelli” and the fauna of the Devonshire 
beds corresponds very closely to that found in the French zone of 
Am. Ma ntelli. 
These arenaceous beds are the “Chalk with quartz grains” of Sir 
H. De la Beche,f but the first adequate description of them was 
that by the late C. J. A. Meyer in 1874,J and in his tabular view 
of the Beer Head section they are numbered as Beds 10> 11, 12. 
He made a large collection of fossils, keeping those from each of his 
beds separate, so that they form a valuable record. From the 
data thus obtained he saw that these beds contained a different 
fauna from that of the Upper Greensand below, and were every 
* Quart. Journ. Geol. Soc., Vol. liv. p. 239. 
fTrans. Geol. Soc., Ser. 2, Vol. ii., p. 110, and Report on the Geology of 
Cornwall, Devon, and West Somerset, p. 237. p 
*Quart. Journ. Geol. Soc., Vol. xxx. p. 369. 
