A, R. Horwood—Aragonite in Middle Lias, Leicestershire. 173 
abnormal faunal phase, and difficulties of correlation immediately 
arise. ‘That is instanced in the lower part of the Rush sequence, and 
in the attempt to trace the subdivisions of the uppermost Visean, 
established in the Midland Province and in the Loughshinny coast 
section, through the totally different facies obtaining in the North of 
England and Scotland. 
The objection, therefore, to a rigid stratigraphical application of 
those corals now regarded as of zonal value, is that the present system 
is dependent on the succession of a number of forms, most of which 
are not in genetic connexion, and of whose true range in time we 
haye at present little or no conception. It must be self-evident that 
it is impossible to estimate the time value of any fossil until the 
evolution of the gens to which it belongs has been worked out, and 
the range of the various mutations ascertained. A system of zoning 
established on such lines may seem an impossible ideal, but it is 
certainly attaimable in some degree. In the meantime Dr. Vaughan 
and his co-workers have, as a rule, utilized the general characters of 
the faunal assemblage as diagnostic of any particular horizon. The 
errors caused by the introduction of species, by favourable conditions, 
at a somewhat different level in the areas concerned, are in this way 
made partially to counterbalance each other. If, in addition, the 
sequence of physical conditions, and therefore of faunal phases, are in 
agreement, then reliance can be placed on the broader aspects of the 
correlations suggested. It is in this way that the unconformity 
within the Limestone in South Wales, and the extensive overlaps in 
North Wales and Yorkshire, have been demonstrated. But the fact 
remains that the finer details of correlation must be open to question 
so long as a zonal scale has to be used in which the genetic sequence 
is so imperfect. That is so even in areas exhibiting a similar 
succession of physical conditions. Even more objection applies to 
precise correlations based on the sudden appearance of certain gentes 
that are not represented in underlying beds. And it is still more 
doubtful whether correlations, except of the most generalized kind, 
can be made by existing methods, between areas exhibiting a serious 
difference in faunal facies. In such cases, it would seem that reliable 
results can best be attained by working out the evolution either of 
the few gentes common to both areas, or of the contrasted gentes, if 
they can be found together in another region. In the meantime, it is 
by the discovery of intermediate links, such as the two interesting 
corals found by Mr. Dixon in South Wales, that a zonal scale, of 
a continuously genetic nature, may eventually be established. The 
frequent discovery of such fossils need cause no surprise; on the 
contrary, it is only to be expected during the progress of research. 
VI.—Own tHe Occurrence oF ARAGONITE IN- THE Mippite Lras oF 
LEICESTERSHIRE; WITH SOME REMARKS ON THE CALCAREOUS CHARACTER 
OF THE SPINATUS BEDS. 
By A. R. Horwoop (Leicester Museum). 
ee: Liassic strata generally are so barren of any minerals of 
importance or of an appreciable size or extent that the record 
of the discovery of one not hitherto known to occur in a crystalline 
