402 REV. THOMAS BROWN ON THE MOUNTAIN LIMESTONE AND 
Beneath this there is a large mass of strata, the calciferous sandstones of his 
nomenclature,—the lower carboniferous of subsequent authors. When I began 
these observations on the Fifeshire coast, I held to this received view, and put — 
the strata into two groups. Down to the bed F all was marine, and marked — 
by me as the Mountain Limestone; below F came the estuarine beds,—the lower 
carboniferous. . 
What first shook my confidence in this classification was the discovery of the 
marine bed L, or rather the results obtained, after a full examination of its fossils. 
Lying 1400 feet down among these estuarine strata, it exhibited not only the same 
fauna with the upper six limestones, but that fauna amply developed. Why 
should it not go into the same group with these upper beds ? 
Next came the question, whether the difference of character between Estuarine 
and Marine could form a safe ground for distinguishing the groups,—it might or — 
it might not be convenient as a local arrangement, but if the groups were so 
formed, would the classification be of any value on a wider area when brought 
into comparison with the strata of other districts? 
But what proved most decisive, was an examination of the great mass of strata 
underlying L. Studying these lower beds on to Fife Ness and northwards, it 
became evident that they had a character of their own by which they might be 
defined and recognised,—that L was really the lowest point or base line of an 
upper group, and that the two were separated by characters more to be depended 
on than the difference between a fresh-water and marine formation. 
The only difficulty in assigning the bed L to the upper group is the fact that 
the mass of strata intercalated between F and L are Estuarine, while these two 
beds are Marine, but there should really be no hesitation in setting aside this 
character as a ground of distinction. Every epoch has its fresh-water and salt- 
water beds contemporaneously formed. Just as at this moment deposits are 
going on simultaneously in our fresh-water lakes like Loch Lomond, in our estu- 
aries, as among the upper reaches of our Firth, as well as in the open sea, all 
representing the same point of time, so the fact that a mass of strata is estuarine 
does not in the least disconnect it with the period of the two limestone bands be- 
tween which it is intercalated. It must be grouped along with them, so that the 
whole mass of the nine limestone bands from A to L, with their accompanying 
strata, must be associated together. 
Now there cannot be any ground even for hesitation as to what portion of the 
geological scale this group belongs to. The fossils which we have enumerated 
not only as a whole, must be referred to the Mountain Limestone, but contain a 
large proportion of the species held to be decisive as characterising that formation. 
Take a list of shells like Productus giganteus, P. semi-recticulatus, P. longispinus, 
Athyris ambigua, A. Royssii, Rhynconella pleurodon, Edmondia unioniformis, 
Bellerophon Urii, B. decussatus,—let these and others similar not only be found, but 

