JUKES 5 S MANUAL OE GEOLOGY. 
141 
lays great stress on the lithological character of particular beds, which, 
on other grounds, are known to constitute important geological horizons. 
“ The extent of single beds is most certainly ascertained in coal mining, in which the 
horizontal (or lateral) extension of beds is followed. Particular beds of coal, or of shale, 
or other rock having remarkable and recognisable characters, are sometimes known to 
spread throughout a whole district. For instance, in South Staffordshire a bed of smooth 
black shale, a little below, the thick or ten-yard coal, is known as the ‘ table batt.’ It 
has a thickness of from two to four feet, and extends over all the greater portion of the 
South Staffordshire coal-field—places where it is known being ten or twelve miles apart 
from each other in straight lines and in different directions. Its original extension was 
probably much greater, since the beds now disappear in one direction bv ‘ cropping out,” 
and are buried in others at too great a depth to be followed. Known beds of coal, with a 
particular designation, such as ‘ Heathen coal,’ extend over still wider ai’eas, and simi¬ 
lar facts occur abundantly in most coal-fields. 
“ Neither is the great extension of single beds confined to those containing coal, but 
is found wherever there are beds of a sufficiently remarkable character to be noticed and 
recognised. A little bed called the bone bed, from its containing peculiar fragments of 
fossil bones, which lies just at the top of the New Red Sandstone of the south of England, 
is found both at Axmouth in Devonshire, and at Westbury and Aust in Gloucestershire—■ 
places full sixty miles apart—the bed itself never being more than two or three feet thick, 
and frequently only as many inches. It is even stated by Mr. Strickland, that he has 
identified this same bed in the form of a white micaceous sandstone up to Defford, in Wor¬ 
cestershire, 104 miles from Axmouth, and at Golden Cliff and St. Hilary in Glamorgan¬ 
shire.—‘ Proceedings of the Geological Society of London,’ vol. iii. pp. 585 and 732. 
Similarly, a bone bed at the junction of the Ludlow rock and Old Red Sandstone, never 
more than a foot thick, and frequently only one or two inches, has been traced at intervals 
over a space of forty-five miles from Pyrton Passage to the banks of the Teme near 
Ludlow. 
“ Whether these beds be absolutely continuous or not over all the intervening spaces, 
these facts are sufficient to prove the uniformity of conditions over very large areas, so 
that wherever deposition took place, it was of precisely the same character. In the case 
of the bone beds mentioned above, the conditions under which they were deposited seem 
to have been so very peculiar that they may perhaps be looked upon as exceptions rather 
than as examples of a rule. It is useful, however, sometimes to know what is possible as 
well us what commonly occurs ; neither, probably, would they be very uncommon if single 
beds were more frequently capable of being traced. 
“When from a single thin bed we come to the examination of a group of a few beds, 
the instances of mineral identity over very wide areas become still more frequent. This 
is especially observable when the group of beds is of a character quite different from the 
larger mass of rocks in which they lie; provided that difference points to a state of 
greater tranquillity or quietness of action, as would a bed of clay occurring in a group of 
sandstone beds, or a bed of limestone or coal occurring in others having a purely-mecha¬ 
nical origin. We may take, as an example, what is called the Bala limestone in North 
Wales. This is a little group of a few beds, rarely exceeding twenty feet in thickness. 
The lowest bed is generally a black crystalline limestone, over which are several beds of 
hard crystalline concretionary and nodular limestone of a gray colour, alternating with 
more shaly or slaty beds. These contain small black nodules, possibly of a coprolitic 
origin.* The softer argillaceous bands wear away more rapidly than the crystalline 
layers, which accordingly stand out in relief like a cornice moulding. By these charac¬ 
ters the Bala limestone may often be perceived at the distance of half a mile on the side 
of a hill, and distinguished from the rocks of hard gritty slate above and below it. It 
extends from near Dinas Mowddwy on the south, to Cader Dinmael, on the north, a dis- 
* “ By ‘ coprolitic’ is meant that they were the ‘ droppings’ of fish or other animals.” 
VOL. V.-EEV. Y 
