188 LIAS OF ENGLAND AND WALES: 
As we know that some species of Ammonites range from Lower 
to Middle Lias, or from Middle to Upper Lias, it is of course 
hazardous to state that any one of the zonal species occurs only 
within certain limits. The evidence, indeed, is opposed to such a 
supposition, and we are sometimes in great difficulty to decide 
whether locally a species has a higher range than usual, or 
whether some of the strata are very much attenuated. This is the 
case in Lincolnshire where we must either group strata yielding 
A. capricornus in the Middle Lias, or conclude that the zone of 
A. margaritatus is present only in a very attenuated form. At 
Staithes in Yorkshire, A. capricornus (small specimens), and 
A. margaritatus occur together in beds that are grouped as 
Middle Lias* ; and I have found the two species in the same bed, 
in the lower part of the Middle Lias of Raasay. 
The difficulties in this and other cases arise from the need of 
fixing some artificial boundary for the purposes of mapping the 
strata and indexing the fossils ; and the only plan to adopt, is to 
group the beds where A. capricornus is prevalent with the Lower 
Lias, and those yielding A. margaritatus with the Middle Lias; 
while admitting that here and there passage-beds may occur in 
the midst of which a doubtful boundary must for convenience be 
drawn. The different groupings adopted by geologists with refer- 
ence to the lower boundary of the Middle Lias, have been 
elsewhere stated (see p. 33). 
That there was no great break between the Middle and Upper 
Lias is indicated by the conformity of the strata. But we have 
a general change from calcareo-arenaceous strata to those of a 
calcareo-argillaceous type ; and there is evidence in the very 
fossiliferous character of the Marlstone, and the great variety of 
species locally preserved, of slow and scanty deposition of 
sediment followed probably in places by an absence of deposition 
for a time.t 
The Marlstone in several localities has yielded forms of an 
Upper Lias character, species identified by some authorities as 
Ammonites annulatus, A. communis, A. crassus, A. Holandrei, 
A. serpentinus, &c., having been obtained from the Rock Bed. 
(See p. 245.) At the present day when more attention is paid to 
minute differences of form, and the species of old authors (unfor- 
tunately) are split up more and more into other so-called species 
or " mutations," great difficulties attend all identifications. It 
is, however, sufficient for our purpose if we recognize the incoming 
of Upper Lias forms during the closing stages of the Middle Lias. 
It may be indeed that the new comers became to a certain 
extent commingled with the earlier forms, in the still unconsoli- 
dated mud of the Middle Lias ; but on this point we can only 
throw out the suggestion, while stating the fact of the preservation 
of Upper Lias Ammonites in the Marlstone. 
* See Wright, Liai Ammonites, p. 97, and Fox-Strangways, Jurassic Rocks of 
Yorkshire, p. 76. 
| See also Judd, Geol. Rutland, p. 65. 
