G
GEOGRAPHY
their walls ; some are circular or elliptical dingles of considerable size, with potato crops, or groves of oak, ilex and aromatic shrubs at the bottom. Whatever soil there may be on the tableland is washed down to the floor of these doline and is of a deep, rich red, the famous red earth of the Carso. Driving or walking over a Dalmatian plateau one is not at once aware of these bosky dells, they are hidden below the general level, but on a fine spring morning the ear is surprised and delighted by the full concert of singing birds rising, apparently, from underground, like music from some invisible choir or organ. These doline, or dingles, follow one another in lines more or less sinuous, and this fact gave the earliest clue to the cause of their formation ; and, further, it is known that, in many cases, they lie above true grottos or caverns, still completely roofed in, with their crowns as yet unbroken, like the famous grottos of Adelsberg and S. Canzian, and those which have only just been discovered and explored on the plateau of Cansiglio in the Veneto. At first, geology adopted the igneous theory of this Karst formation ; the doline were lava bubbles dried and burst or broken in. That theory is now superseded by the aqueous explanation. The doline and caverns under them are now held to be the work of subter-