274 Reviews — Geological Aspects of the Coral-reef Problem. 



small amount ; comparatively large prisms of apatite are numerous. 

 The rock is very fresh ; it shows in places a well-marked proto- 

 clastic structure, and there is a tendency to the formation of reaction 

 rims around clusters of pyroxene. The minerals and the structures 

 of this rock accordingly enable us to identify it beyond question 

 with the laurdalite of the Christiania district." ' 



Foreign erratics are not uncommon on the shores around Flotta. 

 They are the typical erratics of the Orkney boulder-clays, and have 

 mostly been derived — as shown by Drs. Peach and Home— from the 

 Moray Firth area. The association of the laurdalite with these 

 rocks is evidence that it is of the same glacial origin. The alterna- 

 tives to transport by glacial ice are transport by floating ice or 

 transport by a ship as ballast. The first is rendered improbable by 

 the fact that floating objects in the Kattegatt drift to the north 

 along the Norwegian coast. 



Although a close connexion formerly existed between the Orkneys 

 and Scandinavia, there is but little likelihood of a ship coming to 

 Flotta, for it is a very unimportant island. If a ship came there she 

 would use the harbour of Pan Hope near by, and not the open shore, 

 and would probably take in rather than discharge ballast. The warships 

 of the Norsemen frequented Scapa Flow, but the same remarks are 

 true of them, for they went to the northern side of the Flow. There 

 is, then, no reasonable doubt that it is a true glacial erratic, and as 

 such it confirms the accuracy of the track of the ice as shown in 

 the glacial maps, and is of interest as being the first undoubted 

 Scandinavian erratic found in Scotland or in the Orkneys and 

 Shetlands. 



In conclusion, we wish to express our thanks to Dr. Flett for 

 more than encouragement in preparing this paper. 



EEVIEWS. 



I. — The Geological Aspects of the Coral-reef Problem. By 

 W. M. Davis, Ph.D., Sc.D. Science Progress, vol. xiii r 

 pp. 420-44, 1919. 



Ij^OR some years past Professor W. M. Davis has been engaged in 

 . a detailed study of the old problem of the origin of coral reefs, 

 in the light of the new material accumulated by many observers, 

 both European and American. As he very justly points out in this 

 paper, the problem is mainly geological, whereas the majority of 

 writers have regarded it chiefly from the biological standpoint, 

 without a proper appreciation of the geological significance of the 

 facts observed by them, and what is perhaps of still greater 

 importance, the meaning of certain facts that they failed to observe. 

 According to him the most significant of these facts are the embay- 

 ment of the shore-lines of islands encircled by barrier reefs and the 

 existence of unconformable contacts of reef and lagoon limestones 

 with their foundations. In the past attention has largely been 

 directed to atolls, where it is obvious that neither of these features 

 can be observed, but they undoubtedly have a considerable bearing on 



