DIKES PREVIOUSLY DESCRIBED 255 



quartz vein is to be inferred from the remarks upon the subject made by 

 H. Strickland in 1838, in volume v, second series, page 600, of the Trans- 

 actions of the Geological Society. 



Darwin, 1833-1834. While on the voyage of the Beagle, in 1833-1834, 

 Darwin observed three dikes in the harbor above Point Desire, at the 

 eastern side of Patagonia.* 



One of these dikes is " about four feet wide," and " consists of whitish, 

 indurated tufFaceous |matter, precisely like some of the beds intersected 

 by it." 



Darwin is of the opinion that at least two of the dikes were formed by 

 the suction of the waves acting in fissures that opened to the surface, 

 and that the material was thus drawn in from above. 



On page 117 of the same volume, Darwin notes the occurrence of 

 dikes of tuff cutting volcanic tuff, in the Galapagos archipelago. 



Strickland, 1838. In 1838 H. E. Strickland described some dikes of 

 calcareous grit at Ethie, in Rossshire.f The dikes described cut Triassic 

 shales and are composed of very hard sandstone containing some lime 

 carbonate. 



The largest dike is 3 feet thick and at least 200 yards long. Two of 

 the dikes are parallel with the shale beds in which they occur, while 

 the others cut more or less directly across the bedding. From two of 

 them branches a few inches thick are given off. 



Dana, 1838-1842. While on the exploring expedition under the com- 

 mand of Charles Wilkes, from 1838 to 1842, Professor J. D. Dana observed 

 a number of sandstone dikes on the Columbia river near Astoria, Oregon, 

 and gives figures of three of them. | 



One of them is 5 feet wide, is composed of granitic sandstone, and cuts 

 to the top of the bluff of shales in which it occurs. 



Regarding the origin of the dikes near Astoria, Professor Dana says : 



"These pseudo-dikes of sandstone were probably formed after or during the 

 deposition of the sandstone, while the region was yet under water. Fissures were 

 opened, perhaps by the same cause that ejected the basalt of the intersecting dikes, 

 and the fissures were filled at once by the granitic sands, along with an occasional 

 fragment of shale from the walls of the fissure. Their number and irregularity 

 evince that these regions have been often shaken by subterranean forces." 



Buckland, 1839. In 1839 Mr Buckland described sand-pipes in the 

 chalk in the region about London, and reached the conclusion that they 



* Geological observations on the volcanic islands and parts of South America visited during the 

 voyage of H. M. S. " Beagle." Second edition, 1876, p. 438. 



t Trans. Geol. Soc. of London, 2d series, vol. v, p. 599. 



X United States exploring expedition, during the years 1838, 1839, 18-10, ISil, 1842, under the com- 

 mand of Charles Wilkes, U. S. N, Geology, vol. x, pp. 054-656. 



