546 Prof. Garivood — Calcareous Algce. 



texture and pale brownish tint characteristic of other species of this 

 genus, while weathered surfaces frequently show a concen':ric and 

 occasionally a radially fibrous structure. The profusion of this form 

 in Westmorland would lead one to expect its occurrence in other 

 districts where the lowest Carboniferous zones are developed ; but so 

 far as I am aware, no such occurrence has yet been recorded. It 

 may be of interest, therefore, to mention here that a few years ago 

 my friend Mr. P. de G. Benson brought me a specimen of rock 

 from near the base of the succession in the Avon Gorge, which on 

 cutting I found to contain several examples of Solenopora identical 

 with the Westmorland form. 



Mitcheldeania. — The specimens of Mitcheldeania Nicholsoni originally 

 described by Mr. Wethered were obtained from Wadley's Quarry, 

 near Drybrook, Mitcheldean, from the Lower Limestone shales near 

 the base of the succession. Professor Sibly, who has recently made 

 a careful study of the Lower Carboniferous succession in the Forest 

 of Dean,^ has traced this algal layer over a considerable area, and 

 considers it to represent an horizon near the top of K.II. of the Bristol 

 sequence. He has also noted examples of Mitcheldeania at a higher 

 level — namely, in the Whitehead Limestone, an horizon corresponding 

 probably to the base of C.2. During a recent visit to the Mitcheldean 

 district I collected specimens from tlie lower sliales and also from 

 the Whitehead Limestone, and, thanks to Professor Sibly's kind 

 directions, I was able to see numerous sections in which he has found 

 this algal development. There can be no doubt that Mitcheldeania is 

 here an important rock-forming organism at least at two horizons in 

 this district, and that it occurs over a considerable area. Interesting 

 as the development of Mitcheldeania in the Forest of Dean undoubtedly 

 is, its real home in Britain is in North Cumberland and along 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 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 tuhules of Girvanella together with coarser tubes 

 reminding one of the Sphcerocodium deposits of Gotland ; in places 

 again the marked concenti'ic 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 Serpulce 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. 

 1 Geol. Mag., Dec. V, Vol. IX, p. 417, 1912. 



