That these Dene Holes were used as hiding-places when the sails of the Danish Vikings appeared on the horizon is probable enough, but originally they were chalk quarries--some very ancient--for British coins have been found in them. The existence of old lime-kilns near the Chislehurst caves places their origin beyond a doubt. Chalk was largely exported in early times from the Thames to Zealand, whence it was passed through the Low Countries and used in dressing the fields. Altars to Nethalennia, the patroness of the chalk quarries, have been found in the sand on the coast of Zealand; some bear votive inscriptions from dealers in British chalk, and Pliny, writing of the finer quality of chalk (_argentaria_) employed by silversmiths, obtained from pits sunk like wells, with narrow mouths, to the depth of a hundred feet, whence they branch out like the adits of mines, adds, "Hoc maxime Britannia utitur." [Footnote: Roach Smith, _Collectanea Antiqua_, vi. p. 243, "British Archæological Assoc. Journal," N.S., ix.-x. (1903 and 1904).] In Cornwall, moreover, there are what are locally called _fogous_. These are either excavated in the rock with passages leading to the sea or to houses, or else they are built of stone slabs standing erect, parallel and covered with other slabs leading to chambers similarly constructed, and all buried under turf or sand. Of the former description there is a very interesting example at Porthcothan in S. Ervan; of the latter the most remarkable is at Trelowaren. The former may have been excavated by smugglers. An interesting account of the excavation of two caves at Archerfield, in Haddingtonshire, is given in the Proceedings of the Society of Antiquaries of Scotland for 1909. Both caves are natural, but one had been walled up in front, with a doorway and window and with oven; both had paved hearths in the centre, and there was evidence that they had been tenanted some time after the Roman occupation of Britain, as among the fragments of pottery found was some Samian ware. It would appear that both had been inhabited simultaneously, but not consecutively, for a lengthy period, and no doubt can exist that they were mere rock refuges. In a note to the article we read: "On the coast of Island Magee (Ireland) there is a cave, south of the Gobbins, which has been frequently used as a place of refuge. So late as 1798 it was inhabited by outlaws, who constructed a kind of fortification at the entrance, the remains of which still exist." [Footnote: Cree (J. R.), "Excavation of Two Caves," in "Proceedings of the Soc. of Arch. of Scotland," Edin., 1909, vol. xliii.]