190 STRATIGKAPHICAL GEOLOGY 



protected by deposits of marly limestone composed of coral mud 

 and the remains of other organisms. If such an ancient reef were 

 subsequently raised and truncated by marine erosion and finally 

 preserved as part of the earth's crust, we should have a mass of 

 unstratified reef -rock sharply denned on all sides, from the sur- 

 rounding beds, and probably of great vertical height in proportion 

 to its horizontal extension. It would not form a lenticular mass 

 nor would it pass into other kinds of limestone. 



Again the corals which build up such reefs at the present day 

 belong to families which did not exist in Palseozoic seas, and 

 only made their appearance in those of Triassic time. Corals of 

 the single cup-shaped form are not likely to have been reef- 

 builders, and though there were compound corals in the Palaeozoic 

 seas, such as Heliolites, Favosites, Pachypora, and Alveolites, they 

 belong to the Alcyonaria, and though the living genus Heliopora 

 lives on coral-reefs, no reliable inference can be drawn from this 

 fact with respect to the habit and growth of Heliolites and others ; 

 we can only judge from what can be seen of their mode of 

 occurrence in stratified rocks. 



The other group of animals which has been credited with reef- 

 forming capabilities are the Stromatoporidce, which are not corals 

 nor Actinozoa of any kind, but are believed to have been Hydrozoa 

 allied to the modern Millepora. They formed massive hemi- 

 spherical or irregular globular growths of concentric laminae, and 

 varied in size from a few inches to masses of 3 or 4 feet in diameter. 

 They were sometimes so abundant as to be the principal con- 

 stituents of thick beds of limestone, which on account of the size 

 of the component masses are often unstratified ; thus they certainly 

 formed submarine banks, but such banks are not " reefs." 



The beds of coralliferous limestone which occur in the Silurian 

 and Devonian strata of Britain, France, and Germany do not 

 present the aspect of coral-reefs. They are merely lenticular beds 

 or masses, of greater or less horizontal extent, and are often inter- 

 stratified with shales or occur as bands in a mass of crinoidal 

 limestone. Such a mode of occurrence proves that they were 

 formed on a sea-floor in the ordinary way under a certain depth of 

 water. The corals seem in some places to have formed fields or 

 groves, like those of some modern species of Crinoidea, and the 

 resulting limestone is unstratified, but such limestones must not be 

 called reefs because there is no evidence that they were ever built 

 up to the surface of the sea. 



Limestones which have a greater resemblance to coral-reefs 

 occur in Sweden, and some are beautifully exposed in the cliffs of 

 the island of Gottland, where they can be seen to terminate in some 



