30 



NA TURE 



[May 8, 1890 



partly non-calcareous nature, but we even find old reef surfaces 

 exposed to view. The volcanic porphyritic lava, or rather tufa, 

 which was spread over the sea-bottom after the termination of 

 the Buchensteiner period (middle Triassic, Mediterranean pro- 

 vince) covers the deep-sea deposits of earlier date, but leaves 

 the masses of dolomite free. Here and there, as on the Plattkofel, 

 it can be seen overlying the foot of the actual reef precipice 

 and there ending. It is covered by similar dolomite of a later 

 date. 



Many observations by Dana, Walther (" Korallriffe des Rothen 

 Meeres"), and others, have shown that old coral reefs, about the 

 nature of which there can be no doubt, are often dolomitic. 

 The structure of Tertiary coral reefs on the Sinai peninsula, 

 about the origin of which no doubt can be entertained, is actually 

 identical with the structure of some Triassic dolomite I have 

 examined here. I believe myself, for these, and many other 

 reasons, justified in agreeing with Richthofen, &c., and in 

 assuming that many Tyrolese Triassic limestones and dolomites 

 are coralligenous. 



The zones of the Mediterranean Trias differ altogether from 

 the Trias in Germany. Other limits must here be recognized. In 

 this respect I follow Mojsisovics. In the lowest Triassic no 

 coral reefs are observed ; also in the zones corresponding to the 

 German Muschelkalk, we find, although these deposits are 

 usually calcareous, no reefs of any size. It is in the zones 

 distinguished as Buchensteiner, Wengener, and Cassianer- 

 Schichten, that the great massy, unstratified reefs of South 

 Tyrol are met with. The Upper Triassic layers, known as 

 Raiblerschichten and Dachsteinkalk, are in South Tyrol 

 mostly stratified, and in my opinion sedimentary, not coralli- 

 genous. Numerous faults traverse South Tyrol and break up the 

 whole, only slightly folded Triassic system, into numerous plates 

 ( Tafeln) which are elevated on one (usually the northern) and 

 depressed on the other (usually the southern) side. Liassic, 

 Jurassic, and even Cretaceous layers rest on the depressed margins 

 of the plates. Elsewhere these have been entirely removed, and 

 the underlying Triassic reefs, capped with remains of sedimentary 

 Dachsteinkalk and Raiblerschichten, have been laid bare. On 

 the elevated margins along the faults also the Triassic layers have 

 been removed. It is clear that somewhere between the subsided 

 margin of the plates covered with Jurassic deposit, and the elevated 

 margin, laid bare down to the Dyas, there must be places 

 where the erosion has just reached the middle Triassic reefs. 

 Here it is that we find parts of natural reef surfaces exposed. 



The Lower Triassic Werfener Schichten are divided from the 

 middle Triassic by deposits of gypsum, which show that the sea 

 receded after the Werfener Schichten had been formed. After- 

 wards the sea returned, and it is clear that it must have risen at 

 least as high as the later layers are thick, whilst or before they were 

 deposited. The fossils in the sedimentary deposits between the 

 masses of structureless dolomite show that the depth increased 

 during their deposition. These sedimentary deposits alternate, 

 as above stated, at their margins with the dolomite at the foot of 

 the reef precipices. Therefore the dolomite also was formed 

 during the rise of the water, for which I shall henceforth use 

 the more exact expression introduced by Suess — positive shifting 

 of the coast-line. 



The dolomite masses are coral reefs. They have been formed 

 during a period of positive shifting of coast-line, therefore we 

 may assume that the high coral reefs now living and growi ig may 

 have attained their astonishing altitude over the sea-bottom 

 during a period of positive coast-line shifting. The dolomite 

 masses of the Wengener and Cassianer period show no trace 

 of stratification, such as is observed in the lower Muschelkalk 

 and in the higher Dachsteinkalk. Moreover the dolomite has the 

 same perfectly uniform structure from top to bottom ; and on the 

 vertical clifTs produced by erosion, which are often several 

 thousand feet high, no trace of a stratified basal part can be 

 detected. Everywhere the massy dolomite rests on discordant 

 older layers, or — as usually at the reef margin — on the simultan- 

 eously deposited deep-sea sediment. 



I will now proceed to utilize the facts here outlined in criticizing 

 the discussion between the advocates of Darwin's and Dr. 

 Murray's theories concerning the origin of coral reefs. I may say 

 at once that all the phenomena observed in the dolomites of 

 South Tyrol corroborate Darwin's subsidence theory, whereas 

 they do not find explanation if we accept Dr. Murray's. It is 

 the latter, therefore, which requires a closer examination. 



Dr. Murray says that on slight elevations of the sea-bottom 

 calcareous sediment accumulates, whereas in the greater sur- 



rounding depths this is not the case in consequence of the 

 increase of dissolving power of sea-water with increasing depth. 

 I accept this, and I believe that the caps of stratified Dach- 

 steinkalk on the summits of many of the middle Triassic reefs- 

 in South Tyrol have been formed in this manner after the 

 growth of the reefs had terminated. The positive coast- line 

 shifting led to a horizon' al extension of the Triassic Mediter- 

 ranean northward, and a junction with colder northern seas, 

 which caused a cooling of the water in the bay of South Tyrol, 

 and thereby terminated the existence of reef-building corals. 

 The positive coast-line shifting continued, and during its pro- 

 gress the Dachsteinkalk was deposited on the summits of the 

 reefs, whilst the intermediate deeper spaces were left free from 

 calcareous deposit — in accordance with Dr. Murray's view. 

 There is, however, as above stated, nowhere a trace of stratified 

 calcareous sediment forming a basement or nucleus to any one of 

 the dolomite masses. 



Dr. Murray then goes on to say that, as soon as the accumu- 

 lating calcareous sediment has reached the region of co'-al 

 growth, say the 20- fathom line, corals will grow on it, and an 

 isolated atoll rising precipitously, perhaps ten thousand feet 

 from the sea-bottom, will be formed. Against this it must be 

 objected that the soft Globigerina ooze, which forms the whole 

 of the atoll-peak, with the exception of an insignificant coral 

 cap, could never attain such precipitous slopes as the atolls 

 usually have. A slope of 45° or more could never be formed. 

 The fossil deposits of this kind observed in South Tyrol {Ueber- 

 gussschichten of Mojsisovics) nowhere terminate abruptly like 

 the reefs. Neither is a slope of this kind anywhere observed 

 in the rsgion of the Dachsteinkalk. 



Then, according to Dr. Murray, an atoll is formed by the 

 solution of the lime in the centre of the reef. Although the 

 Oolithes discovered in reef regions by Walther show clearly 

 enough that there cannot be any solution other than what is 

 compensated by redeposition, in any enclosed lagoon, I would 

 like to draw attention to the logical discrepancy in this part of 

 Dr. Murray's theory. First, a limestone cone is built, because 

 the lime is deposited more rapidly than it can be dissolved. 

 Then a lagoon is formed because the solution exceeds the 

 accumulation, and this on the same spot, in still shallower and 

 less powerfully dissolving water, and in spite of the relative 

 stagnancy of the water in the lagoon and the limestone material, 

 which is continually washed into the lagoon from the parts of 

 the surrounding reef, which lie above the level of the sea. I 

 think that gives the coup de grdce to Dr. Murray's atoll 

 theory. 



There remains yet something to be said on his ideas con- 

 cerning the lateral growth of reefs. There can be no doubt 

 that there is such lateral growth, and that the band of living 

 coral on the reef margin can advance towards deeper water on 

 a basis of coral fragments which have fallen from the outward 

 growing face of the reef. As the corals near the surface grow 

 more rapidly than those further down, the advancing reef face 

 must ever tend to become overhanging ; parts of the living coral 

 must therefore frequently break off, fall down, and accumulate 

 I below. But there is a limit to this lateral growth which restricts 

 it so considerably that it will by no means explain the formation 

 of masses of dolomite 4000 feet thick, like the Cimon della Pala, 

 for instance ; and far less will it enable an atoll rising 10,000 

 feet or more from the bottom of the sea to extend horizontally. 

 The amount of material available for the formation of a basis 

 whereon the laterally growing corals may find footing is limited, 

 and grows only in proportion to the circumference of a reef. 

 The amount of material required for this purpose grows in a 

 much more rapid proportion because it has to cover the surface of 

 the growing cone. 



Take a simple case. An ordinary straight fringing reef advances 

 on a bottom of 10° inclination straight outward. At a distance 

 of 280 metres from the shore a depth of 50 metres is reached. 

 Further lateral growth is only possible on a talus of coral dSbris. 

 560 metres from the shore the depth is 100 metres. If the talus 

 will Heat an angle of 45°, an amount of 50 cubic metres extended 

 over a surface of 71 square metres will be necessary for the ad- 

 vance of each i metre's length of reef a distance of i metre. 

 This talus is contributed from 50 square metres of growing coral 

 (vertical reef face). At a distance of 5600 metres from the shore 

 the depth will be 1000 metres. Every 50 square metres of grow- 

 ing reef face will have to furnish 950 cubic metres of material 

 to enable the reef to advance I metre. These 950 cubic metres 

 will be distributed over a surface of 1350 square metres. In 



