100 



i^astcropod occur in a sul)f()ssil stale in the consolidati-d rubble l)el(jw the hnie- 

 stone bed. I'rofessor W. Howchin has kindly determined these shells as 

 r>ollir{c]ubryo)i utastcrsi. Cox. Professor F. Wood Jones informs me that similar 

 shells in a livino^ state occur on the surface of the islands to-day. As travertine 

 is a very recent or modern geological formation, the occurrence of a living species 

 in a suhfossil state is not surprising. The travertine limestone that is now 

 present overlying the granite is probably only the remains of what was once a 

 thick deposit of calcareous sand. This has now been eroded, leaving the lime- 

 stone as an indication of its former presence. 



The islands of the West Coast are the remains of the old coast of South 

 Australia that, during the Pleistocene period, was south and west of its present 

 line. The islands and neighbouring portions of the mainland consist of indurated 

 sands resting upon a platform of older rocks. This platform is generally a few 

 feet above or below sea level. It represents, according to Howchin,*^') who has 

 examined the structure of the islands and mainland eastward of Cape Catastrophe, 

 a base level of erosion, probably marine. At some post-Miocene time this plat- 

 form became covered by calcareous sands left by the sea in its line of retreat. 

 Huge dunes must have been formed, for some of the islands, e.g.. Thistle Island, 

 show a thickness of sand over 700 feet. Still more recently the sea returned 

 and is now wasting away the soft wind-constructed sand left in the line of its 

 former retreat. 



There is no reason to doubt that the area which is now Pearson Islands 

 received a deposit of calcareous sand, but it is open to question whether this 

 deposit was continuous over its highest parts. No travertine was found on the 

 hillsides above 200 feet. The opinion is expressed that even at the period of 

 maximum sand covering, the present hilltops (at least, of Pearson Islands), pro- 

 jected as granite outcrops. Such granite outcrops are common on the mainland 

 in Central Eyre Peninsula to-day, and they bear a flora which is different in 

 constitution from that of the porous sandy soil around them. Certain of the 

 plants found on Pearson Island are of this "granite-outcrop" type of flora rather 

 than sand dune. The occurrence of a rock wallaby (Pctrogale pearsoni) that 

 is unknown on the mainland, also indicates that a rocky outcrop has been a 

 feature of the Pearson area for a great length of time. 



ENVIRONMENTAL FEATURES. 

 I. Climatic. 



Rainfall and temperature data were given in a previous paper on the Franklin 

 Islands ^^^ for the two nearest meteorological stations on the mainland. These lie 

 to the north of the Pearson group, so that they represent a more extreme type of 

 climate than that existing there, especially as the Pearsons are islands lying well 

 out in the open sea. llie general climatic conditions, shown in the table for 

 Franklin Island, of winter rainfall and a dry summer, with probably one or two 

 completely rainless months, will therefore hold for Pearson Islands. The tem- 

 peratures are likelv to show a smaller range, days of over 100^ F. or more in 

 the shade are unlikely, as are frosts. No more positive data than those cited can 

 be given, but observations made during a week's stay on the islands suggest 

 some additional featiu^es of the climate. 



The prevalent winds are south-west, and are often of great strength. 

 Shearing action by the winds is well shown by shrubs exposed to their violence. 

 The most striking examples occurred at the head of Main Creek. There the 

 trees of Melaleuca liabnaturorum have a prostrate habit (pi. iv., hg 2). One old 

 tree measuring 24 feet in length and 8 inches in diameter at the base is growing 



t3> Howchin. W., Proc. Roy. Geogr. Soc. S. Austr.. x.. pp. 204-219. 1908. 

 (*) Osborn. T. G. B., loc. cit.. p. 197. 



