XVIII. NOTES ON SOME ROCK-SPECIMENS 

 FROM THE AROA ISLANDS. 



(Plates XXXV— XXXVIII). 



By J. B. ScRiVENOR, Geologist, F.M.S. 



[In August and November, igo6, the Aroa Islands were 

 visited by Mr. H. C. Robinson and an account of the group 

 and of the collections obtained on Pulau Jemor, the largest 

 islet, was published b}' him in the Journal Federated Malay 

 States Museums II, pp. 8-16 (igo6). 



A request having been made to the Museums Department 

 for information as to the geology of the Aroas a third visit was 

 paid to them in February, 1915, to collect rock specimens and 

 to obtain a series of the native rat {E. rattus subsp.,) of which 

 animal insufficient examples had been secured on the former 

 visits. 



It is unnecessary to repeat the description of the islands 

 already given : here it may be added, however, that they are 

 situated near the northern extremity of a 10 fathom area 

 projecting from the Sumatran Coast in long. 100° 33' E. and 

 Lat. 2° 53' N. where they form a compact little group with a 

 number of isolated rocks and islets occurring in the sector 

 N.E. — S.E. of it, at distances varying from 3^ to 7 miles. 

 The main group is fringed by numbers of jagged rocky reefs, 

 many of which are exposed at low spring tides (PI. XXXV., 

 fig. I). 



Contrasted with the numerous forested islets of this 

 region the Aroas are somewhat remarkable on account of their 

 lack of vegetation, a scarcity which Js most pronounced on 

 Pulau Jemor, the north-easternmost and largest of the central 

 islands. On account of their open nature charming views are 

 obtained from the summits of most of them and the exposed 

 reddish earth and rocks add richness to the colour of the scene. 

 Amongst the shrubs in flower in February was the pretty pale 

 pink myrtle, Cynomyrtus tomentosa. 



The rat is the only terrestrial mammal and no bats were 

 seen. No birds besides the common sea or shore species (and 

 the few others which always occur in such situations) except 

 a pitta (P. cyanoptera) and rail {A. phaenicura) were observed, 

 the collection made being practically similar to that secured 

 on the former visit in August, thus showing that the migration 

 season which was at its height in November 1906 had come 

 to an end. A day-flying mosquito was both numerous and 

 active. 



Weathering appears to have taken place most strongly on 

 Jemor, where vegetation is scantiest. The rocks seem to be 

 tilted at a high angle, about 70 or 80 degrees, and to dip from 



