DIKES PREVIOUSLY DESCRIBED 257 



Prestwich also thinks that many of the pipes in the harder hmestones 

 have been formed along the lines of preexisting fissures. 



Kirkby, I860. In 1860 J. W. Kirkby described a number of sand- 

 pipes in the magnesian limestones of Durham.* 



These pipes, or narrow sand and clay filled tubes, extend from a thin 

 sandstone down into an underlying limestone. The longest one men- 

 tioned has a length of 6 feet. Kirkby concludes that the cavities were 

 formed by solution, by waters passing down from the overlying stratum 

 of sand. The sand is overlain by shales which prevent the water, 

 which finds access to the sand, from escaping in any direction except 

 downward. The sand filling the pipes has been derived from the over- 

 lying sand bed, and the filling took place synchronously with the for- 

 mation of the pipe-like cavities. 



A. B. W. and Du Noyer, 1860. In the April Geologist, 1860, A. B. W. 

 calls attention to some irregular vertical veins or thin dikes of dark gray 

 compact limestone, crossing a nearly horizontal bed of red shale in or 

 near the local base of the Old Red sandstone near Templemore.f 



In the July Geologist, 1860,^ Du Noyer suggests as an explanation of the 

 dikes mentioned by A. B. W., that before solidification the shales were 

 elevated above the sea, and sun cracks were formed in them. Afterward 

 the shales were submerged, limestone was deposited, and the cracks of 

 the shale were thus filled. Then, according to Du Noyer, there was a 

 reelevation and removal by erosion of all the limestone except that in the 

 cracks in the shale; then there was another submergence, during which 

 the Old Red sandstone was deposited above the shale in the position it 

 occupies at present. 



Foster and Topley, 1865. In 1865 Messrs C. Le Neve Foster and 

 William Topley described a number of pipes of gravel and brick-earth 

 in the Kentish Rag of the Weald. § They are of the opinion that these 

 pipes are caused by the overlying gravel and clays being let down into 

 cavities formed by the solution and removal of the underlying limestone 

 by acidulated waters. 



Whitney, 1865. On page 40, volume i, of the Geological Survey of 

 California (published in 1865), J. D. Whitney notes the occurrence of 

 dike-like masses of sandstones intersecting shales 7 miles southeast of 

 Corral Hollow, in the mount Hamilton range. In regard to the origin 

 of these dikes Whitney says they " seem to have originated in the filling 



♦The Geologist, London, 1860. On the occurrence of "sand-pipes" in the magnesian lime- 

 stones of Durham, by J, W. Kirkby, pp. 293-298, 329-336. 



IThe Geologist, London, 1860, p. 135. 



t Ibid., pp. 272-273. 



§C. Le Neve Foster and William Topley : On the superficial deposits of the valley of the Med- 

 way, with remarks on the denudation of the weald. Quart. Jour, of theGeol. Soc. of London, vol. 

 21, 1865, pp. 443-459. 



