Tlie Karstlands of Western Yngodavia. 403 



employed for the irrigation of tlie Gacko plain during the hot and 

 dry Herzegovinian summer, with the result that upwards of 2,000 acres 

 of highly fertile marsh land has been brought under intensive 

 cultivation. 



The Popovo polje, on the northern edge of which is situated the 

 ancient walled town of Trebinje, is in. Southern Herzegovina. Its 

 fertile surface covered with vineyards, orchards, and plantations 

 of maize and tobacco, which in the wet season consists of half-river, 

 half-lake, stands out in striking contrast to the wind-swept surfaces 

 of the adjoii^ing inhospitable and rugged karstlands. The polje 

 extends in the direction of the main Dinaric N.W.-S.E. folds for 

 a distance of over 40 miles, and the remarkable parallelism of the 

 sides, together with its steep spurless walls, so well seen around 

 Turkovitsi, are strongly suggestive of its tectonic origin. From the 

 north it is drained by the river Trebinjstitsa, flowing in a south- 

 easterly direction along its floor, while streams bearing the same 

 name enter its southern end around Lastva and drain northwards. 

 The overflow of the Popovo polje takes place westward, under the 

 ridge of massive Cretaceous limestones behind Ragusa, by means of 

 an underground channel, 8 miles in length. Where the waters 

 eventually emerge near Rosgiatto, they constitute the river Ombla, 

 a stream of considerable volume. The confirmation of its being the 

 main outlet of the polje has been clearly proved by observing the 

 sympathy of the ebb and flow of the river Ombla with that of the inland 

 stream, the Trebinjstitsa. Where the Ombla emerges at the surface, 

 under a precipitous limestone wall, nearly 1,.500 feet high, it is a 

 stream fully 40 yards broad, and of sufficient volume to work several 

 small mills near its source, besides also providing a copious supply 

 of fresh water for the town of Ragusa, whither it is carried by means 

 of an aqueduct. From its source to the sea at Gravosa, the river 

 Ombla follows a broad, winding, limestone canyon, 2|- miles long, 

 the precipitous sides of which, covered with cypresses and olives, 

 and dotted with villas, present a charming scene, standing out in 

 vivid contrast with the barren, waterless karstlands which hem it 

 in on either side. 



In Montenegro the more easterly system of jjoljes already referred 

 to, viz. Grahovo, Njegus, Cettinje, and Ljubotin, follow the Dinaric 

 trend, and were probably produced by the same system of fractures 

 that gave rise to the Popovo polje of Southern Herzegovina. Each 

 of these depressions is floored with quaternary sands and clays, while . 

 they are surrounded by steep hills consisting of Triassic dolomites 

 and limestones. The Grahovo polje, which has an area of about 

 6 square miles, is the largest, that of Cettinje (also known as the 

 Cetinjsko polje), in which the chief town of Montenegro is situated, 

 being about half that size. The more easterly group, including those 

 of Nitchitchi, Danilovgrad, and Podgoritsa, are drained by the Zeta 

 river. Of these, that of Nitchitchi is the largest, having a total area 

 of about 20 square miles. 



