Swallow holes we have found to be characteristic features within such districts. When limestones are more nearly pure, as in the Karst region east of the Adriatic Sea, similar features are developed, but upon a grander scale, and certain additional forms are encountered. In place of the sink or swallow hole, there appears the “karst funnel” or _doline_, a deep, bowl-shaped depression having a flat bottom. Such funnels may be 30 to 3000 feet across and from 6 to 300 feet in depth (Fig. 196). Though in one or two instances known to be the result of the break down of cavern roofs (Fig. 197), yet like the swallow holes of other regions these larger funnels appear generally to be the work of solution by the descending waters. Where they have been opened in artificial cuttings along railroads or in mines, the original rock is found intact at the bottom, with small crevices only going down to lower levels. Over the bottoms of the dolines there is spread a layer of fertile red clay, the _terra rossa_, like that which is obtained as a residue when a fragment of the limestone has been dissolved in laboratory experiments. [Illustration: FIG. 197.—Cross section of the doline formed by inbreak of a cavern roof. The Stara Apnenka doline in Carinthia (after Martel).] =A desert from the destruction of forests.=—Between the dolines is found a veritable desert with jutting limestone angles and little if any vegetation. The water which falls upon the surface either runs off quickly or goes down to the subterranean caverns by which so much of the country is undermined. Hence it is that the gardens which furnish the sustenance for the scattered population are all included within the narrow limits of the doline bottoms. Although to-day so largely a barren waste, we know that the Karst upon the Adriatic was in remote antiquity a heavily forested region and that it supplied the myriads of wooden piles upon which the city of Venice is supported. The vessels which brought to this port upon the Adriatic its ancient prosperity were built from wood brought from this tract of modern desert. In the days of Venetian grandeur the fertile terra rossa formed a veneer upon the rock surface of the Karst and so retained the surface waters for the support of the luxuriant forest cover. After deforestation this veneer of rich soil was washed by the rains into the dolines or into the few stream courses of the region, thus leaving a barren tract which it will be all but impossible to reclaim (plate 6 A). [Illustration: FIG. 198.—Sharp _Karren_ of the Ifenplatte Allgäu (after Eckert).]