PRESIDENTIAL ADDRESS, 471 
first place, there can be no doubt from the examples described above that they 
play a very striking part as rock-builders at many different horizons in the 
geological series. At the same time, it is evident that not only are certain forms 
restricted to definite geological periods, but that they had also a wide 
geographical range, and on this account these organisms will often be found 
valuable as zonal indices either alone or in conjunction with various other 
organisms. As an example of this wide distribution we may cite Solenopora 
compacta, which flourished so abundantly during Llandeilo-Caradoc times not 
only in the Baltic area and Scotland, but also in England, Wales, and Canada; 
again, the wonderfully persistent development of the Rhabdoporella facies over 
the whole of the Baltic area at the close of Ordovician times was of so marked a 
character that boulders of these rocks scattered over the North German plain 
can be made use of in tracing the direction of flow of the ice-sheet during glacial 
times. 
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 be used with the 
greatest confidence not only 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 Dibuno- 
phyllum Zone, and the Mitcheldeania gregaria beds in the north of England and 
in the Forest of Dean. ; 
Again the presence of these organisms at a particular horizon furnish us 
with interesting evidence as to the conditions which obtained during the 
accumulation of these deposits. 
At the present day Calcareous Alge 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 Prof. Walther,**;7° where Litho- 
thamnion and Lithophyllum flourish to a depth of from 50-70 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 Algz with oolitic structure and also with dolomite. 
Thus oolites oceur in connection with Solenopora in the lower Cambrian of 
the Antarctic, in the Craighead Limestone at Tramitchell, in the Ordovician 
rocks of Christiania, in the Silurian of Gotland and in the Lower Carboni- 
ferous Limestone of Shap; while in the Jurassic rocks of Gloucestershire and 
Yorkshire it occurs in the heart of the most typical oolitic development to be 
met with in the whole geological succession. Though Mr. Wethered has made 
vut a good case for the constant association of Girvanella tubes with oolitic 
grains there are many cases in which their association cannot be traced. 
M. Cayeux” in writing of a mass of Girvanella from the ferruginous oolites 
of the Silurian rocks of La Ferriére-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 connection with algal deposits at different 
geological horizons appears to have taken place under definite physiographical 
conditions similar to those which obtain to-day in the neighbourhood of coral 
reefs. Such lagoon con:litions would come into existence either during a period 
of subsidence or during a period of elevation, and this is just what we find when 
we examine the periods at which these reefs are most persistent. 
Thus the Girvan Ordovician reef occurred during an elevation which 
culminated with the deposition of the Benan Conglomerate; the Lower Car- 
boniferous ‘ Algal band’ in Westmorland was laid down during the subsidence 
which followed the Old Red Sandstone continental period ; the Upper Girvanella 
Nodular band occurred when the Marine period of the Lower Carboniferous was 
drawing to a close and a general elevation was taking place. Similar conclusions 
could be drawn from other periods recorded above did time permit. 
© Zeitsch. d. deut. Geol. Ges. 1885, p. 229. 
1 Abh. d. Konigl. Preuss. Akad. der Wiss. 1910; see also M. A. Howe, Science, 
N.S. vol. xxxv. No. 909, p. 837 et sqq. 
1 Comptes Rendus, Acad. de Sci. 1910, p. 359. 
