Carboniferous Period in East Scotland. 259 
either depression or more rapid accumulation necessitates short migrations 
to a congenial habitat to escape extinction. 
The upper half of the Milistone Grit shows partial cycles of deposition of 
grit, fireclays, ganisters, and sometimes even thin coal seams. As already 
stated its fossils are of Coal Measure type. 
CoaL MEASURES. 
The Coal Measures in our area are divisible into three groups. 
Lower Coal Measures—The Lower Coal Measures consist of white and 
yellow sandstones, shales, fireclays, coals and ironstones, and attain a thick- 
ness of about 600 or 700 feet. 
These materials show a rythmical cycle of deposition, which, throughout 
the greater part of the column, does not indicate any truly marine or even 
estuarine conditions. This indicates that depression was still proceeding by 
irregular short steps. In the case of the eight-foot coal, however, the late Mr 
Karkby records the presence of Anthracomya wardi, Naiadites (Anthracoptera) 
carinata, Carbonia fabulina, and Spirorbis pusillus (carbonarius) from its 
roof, which indicates a depression bringing on estuarine conditions over a 
land surface. He also records plant remains encrusted with Spirorbis from 
the roof of one of the other seams showing a similar transition. For the 
most part, the lamellibranchs that are found in the shales often overlying the 
coal seams belong to the genus Carbonicola, formerly named Anthracosia, one 
of the family Unionidee of freshwater habit. 
At or near the top of the group, a thin bed of calcareous shale with truly 
marine forms, known as “Skipsey’s Band,” occurs throughout the Midland 
Scottish Coal-fields, and this in Midlothian has yielded Spirifer, Chonetes, 
Productus longispinus, and nautiloids, denoting a widespread and more pro- 
nounced depression of the whole of Southern Scotland which brought in 
marine conditions for a short time over the flat delta of some great continental 
river or rivers. 
The vegetation of the Lower Coal Measures group is so distinctive that 
Dr Kidston has arranged the strata under a special division named by him 
“ Lanarkian.” 
Middle Coal Measwres.—Overlying the dark productive rocks above 
mentioned comes a series of red sandstones, shales, marls and calcareous 
bands, with only a few thin coal-seams. Owing to denudation, comparatively 
small outliers detached from each other are left. In the Midlothian outlier, 
at about 230 feet up from the marine band at the top of the Lower group, 
another thin estuarine or marine band occurs which has yielded Lingula, 
Productus, a pectiniform lamellibranch, and a nautiloid shell. In the Fife 
VOL. XIX. iS) 
