22 THE GEOLOGICiiL AGE OP CENTRAL AND WEST CORNWALL. 



occurrence of rounded particles being very local and exceptional. 

 Grenerally they are held together by a somewhat calcareous 

 cementing material, so that they are readily decomposed, the 

 result being the production of the deepest, richest, and most 

 productive soils in the district. Where they lie flat, or at low 

 angles, they are usually denuded away into broad shallow valleys, 

 so as to form very characteristic features in the regions in which 

 they occur.*' In a few places, however, where these sandstones 

 are crossed by mineral lodes or siliceous cross-courses, they are 

 found to be silicified to considerable distances on either side of 

 the cross-vein. In such places, as at Bissick in Ladock parish, 

 Treworgans in Probus, and Treworgans in St. Erme, the rock is 

 very hard and durable, and forms an excellent road-metal. In a 

 few instances rocks so silicified stand up above the general 

 surface of the country, and form prominent features in the 

 landscape. "^^ Whether hard or soft, — fine or coarse in grain, — 

 these sandstones almost invariably contain angular fragments of 

 dark-slate ; never in the least degree rounded, and exactly re- 

 sembling some thin slaty bands, which occur interbedded with 

 the sandstones. Some are so thin, and at the same time so very 

 irregular in shape, that they cannot have been subject to the 

 smaEest friction on the shore. They are too large to have been 

 floated out on the surface of the water, and must, I think, have 

 been blown by the wind from cliffs near by into the sea in 

 which the sandstones were being laid down. 



Cross-bedding is very frequent if not invariable in the thicker 

 beds of sandstone, and, as in many similar instances seems to 

 indicate that they have been deposited very rapidly. 



The Ladock beds, although pierced by many intrusive rocks, 

 have a remarkably regular east and west strike, and often a 

 pretty considerable southern dip. They are, however, rolled 

 over and crumpled together to a considerable extent, at least 

 three anticlinal curves being observable in the Ladock valley, 

 within the space of less than four miles, as shown in the accom- 

 panying section, fig. 1, plate a. After a great deal of trouble, and 



* The home-fields, orchards, and gardens, of the older farms, are often planted 

 on these decomposed and disintegrated sandstones. 

 f The " Halebote rock " in the parish of Creed is composedjof this^sandstone. 



