_- oes 
PRESIDENTIAL ADDRESS. 467 
we shall have to speak of the form from the lower horizon at Mitcheldean as 
M. Nicholsoni, and that from the Whitehead Limestone as J. gregaria. 
Frequently associated with the latter is a curious festoon-like growth, while in 
the lower horizon a Spongiostroma-like structure is often found in the matrix of 
the rock between the larger tubes of M. Nicholsoni. Some years ago Mr. 
Wethered ** also recorded a similar form of Mitcheldeania from the base of the 
middle limestones of the Avon Gorge, while I have myself collected nodules con- 
taining specimens apparently referable to M. Nicholsoni from the Modiola Shales 
near the base of the Bristol succession. Interesting as the development of 
Mitcheldeania in the Forest of Dean undoubtedly is, its real home in Britain is in 
north Cumberland and the Scottish Border, where it flourished to a remarkable 
extent in the shallow water lagoons which spread over so large an area in the 
north of England during early Carboniferous times. Over the greater part of 
north Cumberland and the east of Roxburgh we find a remarkable develop- 
ment of algal limestones in the formation of which Mitcheldeania plays a 
very important part. It is met with especially at two horizons, an upper 
one, lying immediately below the Fell Sandstone, and a lower one in the middle 
of the underlying series of limestone and shales. The lower horizon is 
especially interesting on account of the thick masses of limestone composed 
almost entirely of algal remains. Though Mitcheldeania forms the basis of this 
reef-like development, it is accompanied by other algal forms, especially 
bundles of the minute tubules of Girvanella, together with coarser tubes remind- 
ing one of the Spherocodium deposits of Gotland. In places again the marked 
concentric coatings resemble certain forms of Spongiostroma. The substance of 
the reef has frequently formed round the remains of Orthoceratites—indeed, the 
chief layer is usually associated with remains of these Cephalopoda. With other 
layers occur tubes of Serpule and remains of Ostracoda. In addition to the 
limestone of this massive reef, abundant nodules of Mitcheldeania lie scattered 
through the calcareous shales both above and below. 
The upper layer, from which Nicholson obtained his type specimen of 
M. gregaria at Kershope Foot, forms a compact limestone several inches thick. 
It is made up of small spheroidal nodules about half-an-inch in diameter, and 
occurs a short distance below the Fell Sandstone. It can be traced over the whole 
of north Cumberland and north-west Northumberland from near Rothbury on the 
east to the Scottish Border at Kershope Foot, and from the head waters of the 
Rede in the north to the Shopford district in the south. This layer must there- 
fore have been originally deposited over an area of at least 1,000 square 
miles. The horizon of the upper band is almost certainly that of the C. zone 
of the Bristol sequence.**, ’ It is quite possible, therefore, that it is contem- 
poraneous with the Whitehead limestone of Mitcheldean. This supposition 
receives support from two other pieces of evidence. In the beds underlying 
the Mitcheldeania gregaria band in north Cumberland occur calcareous nodules 
largely made up of tubes of Serpula—an organism which is completely absent 
from the Westmorland succession, but which is reported by Prof. Sibly from 
the lower limestone shales containing Mitcheldeania in the Forest of Dean 
district. Again, this upper algal layer in Northumberland and Cumberland is 
almost immediately overlain by the Fell Sandstone series, while the Whitehead 
limestone at Mitcheldean passes immediately upwards into a sandstone, the 
Drybrook Sandstone of Prof. Sibly, which was originally correlated with the 
Millstone Grit, but was shown by Dr. Vaughan in 1905 to belong to the Lower 
Carboniferous series. It would be interesting if further researches should prove 
the existence of a former gulf at the end of Tournaisian times, running from the 
Forest of Dean to the east of North Wales, through North Cumberland to 
the southern slopes of the Cheviot Isle, with a branch given off eastward into 
Westmorland. 
In any case, it is a remarkable fact that we have a great development of 
algal deposits at this period in Gloucestershire, Westmorland, Lancashire, north 
Cumberland, and Northumberland. 
°5> Brit. Assn. Rep., 1898, p. 862. 
°8 Geology in the Field, 1910, pt. 4, p. 683. 
7 Q.J.G.8., vol. LXVIII., 1912, p. 547. 
HH 2 
