552 Prof. Garwood — Calcareous Algce. 



To take examples nearer home. The ' Ortonella Band ' found 

 throughout Westmorland and North Lancashire near the summit of 

 the Tournaisian occurs so constantly at the same horizon as to 

 constitute one of the most valuable zonal indices in the succession in 

 the North- West Province, and can not only be used with the greatest 

 confidence for correlating widely separated exposures, but it has also 

 afforded valuable evidence of tectonic disturbances. Other examples 

 are supplied by the ' Girvanella Nodular Band ' at the base of the 

 Upper Dibunophyllum Zone, and the Mitcheldeania gregaria beds in 

 the North of England and in the Porest of Dean. 



Again, the presence of these organisms at particular horizons 

 furnish us with interesting evidence as to the conditions which 

 obtained during the accumulation of the deposits in which they occur. 



At the present day Calcareous Algae flourish best in clear but 

 shallow water in bays and sheltered lagoons. As a good example we 

 may take the Algal banks in the Bay of Naples, described by 

 Professor Walther,' where Lithothamnion and Lithophyllum flourish 

 to a depth of from 50 to 70 metres. There is seldom any muddy 

 sediment on these banks, though detrital limestone fragments are 

 widely distributed. Another interesting point is the constant 

 association of fossil Calcareous Algge with oolitic structure and also 

 with dolomite. 



Thus oolites occur in connexion with Solenopora in the Lower 

 Cambrian of the Antarctic, in the Craighead Limestone at Tramitchell, 

 in the Ordovician rocks of Christiania and the Silurian of Gotland, and 

 in the Lower Carboniferous Limestone of Shap ; while in the Jurassic 

 rocks of Gloucestershire and Yorkshire this genus occurs associated 

 with the most typical oolitic development to be met with in the whole 

 geological succession. Though Mr. Wethered has made out a good 

 case for the constant association of Girvanella tubes with oolitic 

 grains there are many cases in which this association cannot be 

 traced. M. Cayeux^in writing of a mass of Girranella from the 

 ferruginous oolites of the Silurian rocks of La Ferriere-aux-Etangs 

 expresses his opinion that Girvanella encrusts the oolite grains but 

 does not form them, and that it is really a perforating alga of 

 a parasitic nature. 



The presence of dolomites in connexion with algal growths at 

 different geological horizons appears to show that the beds have 

 accumulated under definite physiogi'aphical conditions similar to 

 those which obtain to-day in the neighbourhood of coral reefs. Such 

 lagoon conditions would tend to come into existence during periods 

 of subsidence or elevation, and this is just what we find when we 

 examine the periods at which these reefs are most persistent. 



Thus the Girvan Ordovician lagoon-phase occurred duringan elevation 

 which culminated with the deposition of the Benan Conglomerate; 

 the Lower Carboniferous ' Algal band ' in Westmorland was laid 

 down during the subsidence which followed the Old Red Sandstone 

 continental period, while the Upper Girvanella Nodular band occurred 



^ Zeitsch. cleut. Geol. Ges., 1885, p. 229. Abh. Kijnigl. Preuss. Akad. 

 Wiss., 1910. 



^ Comptes Eendus Acad, de Sci. 150, 1910, p. 359. 



