Prof. Garwood — Calcareous Algce. 547 



The upper horizon 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 

 a,bout 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 headwaters of the Rede in the north to the Shop- 

 ford district in the south. This layer must therefore 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 

 contemporaneous 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 

 SerpulcB — an organism which is completely absent from the West- 

 morland succession, but which is reported by Professor Sibly from 

 the lower limestone shales containing llitcheldeania 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 

 Professor Sibly, which was originally correlated with the Millstone 

 Grit, but was shown by Dr. Yaughan 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 rem ark able. 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 glahristria 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 tufts 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, the bifur- 

 cations 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. 



^ Geology in the Field, pt. iv, p. 683, and Q.J.G.S., vol. Ixviii, p. 547, 

 1912. 



