Prof. Garwood — Calcareous Algm. 545 



Obviously, we have here an intrusion of the same nature as the 

 diorite iitrusion of Garabal Hill, but differing from the latter by 

 the fact that part has undergone more rapid cooling, and hence has 

 formed tiie diorite-porphyry. Analogous rocks, hovrever, occur in 

 the Strath Dubh TJisge valley, where a few dykes of very decomposed 

 felspar-porphyry outcrop. An occurrence of diorite apparently 

 passing to porphyrite has been described from Loch Melfort,' but 

 the Geological Survey are of the opinion that the porphyrite is au 

 earlier intrusion which has been altered by the diorite. In the 

 island of Colonsay intrusions of augite-diorites have been found, 

 grading to lamprophyres on the margins.^ The Junctions of the 

 different types in Glen Lednock are very obscure, and hence the 

 relations between the rocks cannot be ascertained. 



IV. — On the Important Part plated bt Calcareous Alg^ at 

 CERTAIN Geological Horizons, with Special Eeference to the 

 Paleozoic Eocks. 



By Professor E. J. Garwood, M.A., V.P.G.S.^ 



[Concluded from the November Number, p. 49S.) 



(WITH FOLDING TABLE II.) 



Soleno'pora. — The discovery of this genus in the Lower Car- 

 boniferous rocks of Westmorland is of considerable interest, as 

 its occurrence here gives us some insight into the history of its 

 wanderings between the time when we last recorded it in the Got- 

 landian rocks of the Baltic area and its subsequent reappearance in the 

 Lower Oolite of Gloucestershire. Whether it lived in the Baltic 

 area during the Devonian and Carboniferous periods is, however, still 

 unknown. The fact of its occurrence in the Caradoc, Carboniferous, 

 and Jurassic rocks of the British Isles would appear to point to its 

 existence not far o:ff during the intervening periods. 



In Westmorland and Lancashire Solenofora occurs in considerable 

 abundance near the local base of the Lower Carboniferous rocks, and 

 contributes largely to the formation of limestone deposits. It is 

 present wherever the lowest beds of the succession are exposed, as 

 at Shap, Ravenstonedale, and Meathop, and must formerly have 

 flourished over a considerable area. 



Though bearing a general resemblance, both in hand specimens and 

 in microscopic structure, to the Ordovician and Jurassic forms, it has 

 recently been shown by Dr. G. J. Hinde to be specifically distinct.* 

 It occurs as small, spheroidal nodules up to an inch in diameter, 

 having a markedly lobulate outline embedded in compact and usually 

 dolomitic limestones, and it is occasionally associated with oolitic 

 structure. When fractured it exhibits the compact porcellanous 



^ Flett in Geology of Oban and Dahnally (Mem. Geol. Surv. Scotland), 

 1908, p. 69. 



* Wright & Bailey, loc. cit., pp. 33-4. 



^ Edited and slightly abridged with the author's permission from the original 

 Address as delivered at Birmingham, before Section C at the British Association 

 meeting. 



* Geol. Mag., Dec. V, Vol. X, p. 289, 1913. 



DECADE V. — VOL. X. — NO. XII. 35 



