GEOLOGICAL OBSERVATIONS IN FIJL 19 



is intnulod hy an andrsitic dike, it was inferred that the volcanic rocks 

 were erupted after the uphft and dissection of the hinestone. 



It is dithcult to correlate the history of the Yasawa group with that 

 of Yiti Levu. In discussing the origin of the great l)arrier reef, it 

 was stated that the lagoon was probably an old feature and had 

 experienced a number of uplifts and depressions. The spine-like 

 mass of limestone in Yasawa-i-lau is but a remnant of a formation 

 once much more extensive. The destruction of so much elevated 

 limestone can only mean that prolonged erosion has affected the area 

 now occupied by the broad lagoon. 



There is no reason to suppose that the Yasawa limestones are older 

 than those of the Lau group, which will be described later. The 

 limestones of Lau were elevated during, or soon after, the close of the 

 Pleistocene period. The topography of the islands of the Yasawa 

 group is much younger than that of the coastal hills of Viti Levu 

 across the lagoon. The andesites forming the latter hills are assumed 

 to be contemporaneous with the andesites underlying the sediments 

 of the coastal plain. 



These observations suggest that the andesites, extruded after the 

 uplift and folding of the interior sediments of Viti Levu, were eroded, 

 submerged, and overlain by coralliferous limestones and marls through- 

 out the lagoon area; that during the Pleistocene these limestones 

 were warped upward; and that they were then eroded and covered 

 by ash and agglomerate during a more recent (Late Pleistocene or 

 earl}- Recent) period of vulcanism and have since been partly sub- 

 merged, as shown by the embayed coast lines of the Yasawa Islands. 



Outline of the Geological History of Western Viti Levu. 



It should be understood that the paleontological evidence from the 

 South Sea islands is so incomplete that the dates assigned to the 

 various e\'ents are only approximations. The history, set down in 

 order, is as follows : — 



1) An old land of slates and red sandstones was intruded by a 

 batholithic mass which solidified as gabbro, diorite, and granite. 



2) The mountain block then formed was deeply eroded and the 

 igneous rocks exposed. 



3) Over the eroded surface andesitic flows were poured out. 



4) The surface of these flows was deeply eroded and submerged. 



5) A coralliferous conglomerate was laid down on the submerged 



