The Karstlands of Western Yugoslavia. 401 



represent the former siibterraneau course of tke Poik river, wliicli at 

 the present time follows a lower hidden course, which can be followed 

 in a boat for more than a mile, mainly along major joints in the massive 

 Cretaceous limestones. The overlying limestone at the surface is 

 studded Avith innumerable circular pits, known as " ponors," which 

 in wet seasons considerably augment the river Poik where it emerges 

 as the river Unz at Planina. There are five main caverns at Ad els- 

 berg which of late years have attracted a great deal of attention 

 from visitors, and for whose benefit they are now illuminated with 

 electric light in the summer months. One of the most spacious, the 

 " Franz- Joseph-Elizabeth " grotto, measures over 200 yards in 

 length and breadth, and has a height of more than 100 yards. Its 

 unusually large stalactites and stalagmites are of considerable 

 interest. 



The caverns, Avhich are so common a feature of the Dalmatian 

 islands, have long been famed for their extreme beauty. Many occur 

 about sea-level, and owe their charm to the deep-blue iridescent 

 colour of the water, doubtless the result of their being wholly in 

 massive dolomite or limestone. Among the more important 

 maritime grottoes, those of the island of Lissa stand pre-eminent ; 

 while the limestone grotto of Busi, a rocky islet off Lissa, has long 

 been noted. On the mainland to the east of Ragusa-Vecchia, 

 and south of Ragusa, is the famous grotto of Esculapius. 



POLJES. 



Reference has already been made to the distinctive land forms of 

 this region known as poljes. The polje (in Serbian : a field or 

 cultivated area) consists of an elongated valley with a flat floor and 

 steep sides ; usually with an internal system of drainage, so that in 

 wet seasons it is not infrequently flooded and becomes an inland 

 sea or marsh. Poljes are usually bounded by scarp faces of 

 Cretaceous or Triassic limestones and on their floors lie newer 

 Tertiary sediments, which being arenaceous constitute the only 

 fertile areas on the karstlands. WTiile their outlines are invariably 

 governed by the predominant Dinaric structure of the country, 

 some appear to be the direct result of longitudinal fractures, con- 

 stituting, in fact, true rift-valleys or Graben, while others are 

 essentially the result of denudation under karst conditions, deter- 

 mined only in part by diastrophic processes. Several of the smaller 

 so-called poljes of Southern Herzegovina, on the other hand, show 

 no clear evidence of such an origin, and appear to result from the 

 collapse of underground watercourses. To such depressions it 

 would perhaps be better to restrict the use of the local Bosnian 

 term " uvalas ", such minor depressions being also known in 

 Montenegro as " valas ". The more important Balkan poljes are 

 those of Livno, Glamoc, Kupres, and Duvno, in Western Bosnia ; 

 and in Herzegovina those of Nevesinje, Popovo, and Gacko. The 

 Montenegrin poljes belong to two parallel systems following the 



VOL. LIX. — NO. IX. 26 



