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to the trend of the coast, in 

 fig. 6. Lagoon-phases are regarded 

 as coastal deposits, and the lagoon- 

 area, consequently, is shown in the 

 figure as a coastal shelf, for reasons 

 which appear later (pp. 525-28). 

 The area is represented as bordered 

 by rapidly deepening water, to accord 

 with the general profile of recent 

 coastal shelves. 



That extremely - shallow marine 

 areas, such as are here postulated, 

 have formerly existed is beyond 

 question, for the deposits in some 

 cases may be interpreted without 

 hesitation. Under the names of 

 ' Modiola and Posidonomya phases ' 

 some of these deposits, of Avonian age 

 and calcareous facies, have already 

 been described by Dr. Vaughan, 1 

 and the object of this note is, partly 

 to bring out the distinctive cha- 

 racters of these phases, partly to ex- 

 tend the concept to rocks of cherty 

 facies. Lagoon-phases, therefore, 

 would include both Modiola and Po- 

 sidonomya phases and equally shallow 

 deposits of other ages and facies. 



But, although lagoon-areas have 

 undoubtedly existed, it is probable 

 that their extreme shallowness and 

 wide extent together would not 

 sufficiently account for the general 

 absence from, them of strong currents, 

 which, as we shall see, has been 

 their distinguishing feature. Appa- 

 rently the rise and fall of the tide, 

 which, if considerable, would inhibit 

 lagoon-conditions on an open coast, 

 has been either so small as to be 

 negligible, or has been rendered 

 inoperative by subaerial barriers, 

 which may have included thickets of 

 vegetation such as those of mangrove- 

 swamps, along the margins bordering 

 deeper water. 



1 These are tabulated by him in Table II 

 of his report on the faunal succession in 

 the Avonian, Rep. Brit. Assoc. (Winnipeg) 

 1909, p. 190 ; reference will presently be 

 made to his description of the development 

 of these phases in the Avon section. 



