1916-17.] The Hurlet Sequence and the Abden Fauna. 201 
XII. Stratigraphical Comparisons and Considerations. 
We now pass to certain stratigraphical comparisons and considerations 
which will be best understood by an examination of the series of compara- 
tive vertical sections given in the Plate. It will be noticed that in the 
first five of these — that is, the Hurlet, Campsie, Corrieburn, Calderwood 
Glen, and Carluke Sections — the datum line has been drawn at the Blackbyre 
Limestone. A similar position has been taken for the datum line in the 
St Monans, Aberlady Bay, and Dunbar Sections in the East of Scotland. 
In the remaining sections, which include those at Skolie Burn, Charlestown, 
Kinghorn, and Bilston Burn, the Hurlet Limestone with its underlying 
Alum Shale fauna has been taken as the datum line. The reason for this 
will be shown presently. 
The outstanding feature of the first five sections, which we must 
constantly keep in mind, is the existence of an important unconformity 
and overlap at the junction of the Lower Limestone Series with the 
underlying contemporaneous volcanic rocks. In the Hurlet district the 
sediments lying below the Blackbyre Limestone attain a thickness of close 
on 1000 feet, and though they consist largely of barren sandstones and 
ashy muds and shales, a marine horizon is occasionally to be met with. 
If we turn to the section at Campsie, it will be found that in the Craig- 
maddie district there is a great thickness of sediments, estimated to be 
upwards of 600 feet, lying between the lavas and the Blackbyre Limestone. 
At Corrieburn, only some 4 miles further to the east, the sediments between 
the Blackbyre Limestone and the lavas is reduced to less than 70 feet. 
In the Calderwood Glen and Carluke districts similar variations indicative 
of the existence of a considerable unconformity and overlap have been 
established. 
In addition to the above direct stratigraphical evidence of unconformity 
and overlap there is a pronounced colouration of the sediments up to the 
horizon of the Hurlet Limestone, due to the presence of decomposed volcanic 
material washed off the old land surface. After we reach the top of the 
Hurlet Limestone we find that the character of the limestones undergo 
a sudden change. The succeeding limestones are generally of a darker 
colour and of a more calmy nature than the underlying ones, and the 
terrigenous sediments are no longer tinged with the green or purple ashy 
material from the old volcanic platform, but consist for the most part of 
dark shales charged with fossils, with which are associated Clayband 
Ironstones. 
When we cross the watershed to the basin of the Forth we find that 
