C.—GEOLOGY. 85 
That depth is not the determining factor is clearly indicated by 
another section taken in the Sams ¢ Belt :— 
Community Depth Character of Bottom Temp. | 
1. Macoma Community (No 8 metres | Pure sand 18.5° C. | 
Echinoderms) 
2. Calearea Community . . | 18 metres | Light mixed clay 10.1° C. 
and sand 
3. Rich Modiola Echinoderm 18 metres | Coarse gravel with 10.3° C. 
Community sand, clay, pebbles. | 
This Macoma community is very well known, as it occurs in all 
the more sheltered waters of the Danish Fjords, and can be directly 
observed and examined at low-water. It is seen to present many 
facies, and to vary greatly according to whether the bottom is sandy, 
stony, muddy, or soft, and according to whether it is exposed to the 
action of currents and to varying conditions of temperature and salinity. 
The fauna in bulk, apart from those characteristic species which belong 
to the community as a whole, varies considerably in different localities, 
and, as the author of the Report expresses it, “the real matter for 
wonder is that there are some species common to all these localities 
and different conditions.’ 
The difference between the characteristic animals of the communi- 
ties living in waters of different depth is so great that none of the 
animals are common to both. This does not mean that no species are 
common to shallower and deeper waters, but that no characteristic 
species as such. 
_ Now the interest for the geologist in all this lies in the fact, as, 
Petersen has pointed out, that these ‘characteristic animals’ are 
closely akin to the ‘ characteristic fossils ’ of the geologist, and we may 
ask ourselves whether the variations which can be seen to exist in the 
contemporar.eous shallow-water fossil assemblages of past ages may 
not be recognised as brought about by the same factors as those that 
can be seen operating to-day. 
Our ancient Lower Paleozoic faunas were composed in the main 
of trilobites, brachiopods, and corals, both solitary and reef-building. 
The distribution of coral reefs at the present day is governed by three 
cardinal factors * :— 
1) Uniformly warm waters, the temperature of which does not | 
fall below 22° C. on an average throughout the year ; ) 
(2) A depth not exceeding 14 fathoms; 
(3) Clear waters, i.e. those free from mud in suspension ; 
and there is every reason to believe that formations containing the 
remains of coral reefs were laid down under very similar conditions to 
these; hence reef-building corals might be expected to have flourished 
2 1923, Potts, F. A. ‘The Distribution of Coral Reefs.’ School Science 
Review, Feb. 1923. 
