SEPTEMBER 25, i913] 
NATURE 
119 
from the lower horizon at Mitcheldean as M. Nichol- 
soni, and that from the Whitehead limestones as 
M. gregaria. Frequently associated with the latter 
is a curious festoon-like growth, while a Spongio- 
stroma-like structure is often found in the matrix of 
the rock between the larger tubes of M. Nicholsont. 
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 containing specimens apparently 
referable to M. Nicholsoni from the Modiola Shales 
near the base of the 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 development 
of algal limestones in the formation of which  Mitchel- 
deania plays a very important part. They are 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 lime- 
stone and shales. The lower horizon is especially 
interesting on account of the thick masses of lime- 
stone composed almost entirely of algal remains. 
Though Mitcheldeania forms the basis of this reef- 
like development, it is accompanied by other algal 
forms, especially bundes of minute tubules of Gir- 
vanella and coarser tubes reminding one of the 
Spherocodium 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 Orthocera- 
tites; indeed, the chief layer is usually associated with 
remains of these Cephalopoda. With other layers 
eccur tubes of Serpule, and others again with 
ostracod remains. In addition to the limestone of 
this massive reef, abundant nodules lie scattered 
through the calcareous shales above and below. 
The upper layer, which includes examples from 
Nicholson’s type locality, 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 1000 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 contemporaneous with the Whitehead lime- 
stone of Mitcheldean. This supposition receives sup- 
port 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 Northumber- 
land 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 Mill- 
stone Grit, but was shown by Dr. Vaughan in 1905 
Age ““Geology in the Field,” pt. 4, p. 683, and Q. G. S., vol. 73, 1912, 
47- 
NO. 2291, VOL. 92] 
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 to the east of the North Wales Island, 
from the Forest of Dean through north Cumberland 
to the southern slopes of the Cheviot Isle, with a 
branch given off towards 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. 
Ortonella. 
This form, as already mentioned, occurs in great 
abundance in the algal band in the “Athyris glab- 
ristria zone”’ of the north-west province It is found 
in spherical nodules up to the size of a small orange. 
In microscopic sections it resembles Mitcheldeania in 
so far as it consists of a series of tubes growing out 
radially from a centre. It differs, however, from this 
genus in many important respects. All the tubes are 
approximately of the same size, and there is no 
evidence of alternating coarse and fine turfs arranged 
concentrically, as in the case of Mitcheldeania. 
Further, the tubes are not undulating as in that 
genus, and therefore in thin slices lie for a long 
distance in the plane of the section. They are much 
more widely spaced and show marked dichotomous 
branching, bifurcation making a nearly constant angle 
of about 40°, and there is a strong tendency for the 
branching to take place in several tubes at about the 
same distance from the centre of growth, producing 
a general concentric effect in the nodule. 
The diameter of the tubes is decidedly less than 
those in Mitcheldeania, being usually little more than 
half the size of the tubes of M. gregaria. The nodules 
of this genus occur in great profusion, contributing 
largely to the formation of the shaley dolomite at 
the base of the “P. globosus band” throughout the 
Shap, Ravenstondale, and Arnside districts and West- 
morland and Lancashire. 
In addition to these genera there occur also two 
other encrusting calcareous growths which require 
mention. The first of these appears in thin sections 
in the form of a festoon-like growth, surrounding 
fragments of calcareous alge, especially Mitcheldeania 
and Ortonella. I have met with it abundantly in the 
“Algal band” in the north-west of England, but it 
also occurs: not infrequently associated with Mitchel- 
deania in the Whitehead limestone in the Forest of 
Dean, while a similar structure occurs associated with 
Mitcheldeania gregaria in north Cumberland. 
Although the exact nature of this growth is still 
undecided, I mention it here on account of its in- 
variable association with undoubted calcareous alge. 
The other deposit is the form already alluded to 
under the term Spherocodium. which I have found 
forming considerable masses of rock in many dis- 
tricts where the Lower Carboniferous beds are ex- 
posed; not only in Westmorland and north Cumber- 
land, but also in the Bristol district, the Forest of 
Dean, and South Wales. 
Foreign Carboniferous. 
From its general similarity to the British deposits 
we might expect to find examples of an algal develop- 
ment in some portion of the Belgian Lower Car- 
boniferous succession. As already mentioned, large 
masses of encrusting calcareous deposits have been 
described by Giirich *® from the Visean limestones of 
the Namur basin as Spongiostroma, &c., which, 
though referred by him to the Rhizopoda, may very 
29 “Les Spongiostromides du Viséen de la Province de Namur. Mem. 
du Musée Roy. d’Hist. Nat. de Belgique,” t. iii 1906. 
