Sandstone Dikes observed in other Localities. 



On the voyage of the Beagle in the winter of 1833-34 Darwin observed 

 three vertical dikes composed of fragmental material some miles up the 

 harbor above Port Desire, Patagonia. He says : 



" The first is straight, with parallel sides, and about four feet wide ; it consists of 

 whitish, indurated tufaceous matter, precisely like some of the beds intersected by it. 

 The second dike is more remarkable; it is slightly tortuous, about eighteen inches 

 thick, and can be traced for a considerable distance along the beach. It is of a pur- 

 plish-red or brown color, and is formed chiefly of rounded grains of quartz, with 

 broken crystals of earthy feldspar, scales of black mica, and minute fragments of clay 

 stone porphyry, all firmly united together in a hard sparing base. The structure of 

 this dike shows obviously that it is of mechanical and sedimentary origin ; yet it . 

 thinned out upward and did not cut through the uppermost strata in the clitt's. This 

 fact at first appears to indicate that the matter could not have been washed in from 

 above ; but, if we reflect on the suction which would result from a deep-seated fissure 

 being formed, we may admit that if the fissure were in any part open to the surface 

 mud and water might well be drawn into it along its whole course. The third dike 

 consists of a hard, rough white rock, almost composed of broken crystals of glassy 

 feldspar, with numerous scales of black mica, cemented in a scanty base. There was 

 little in the appearance of this rock to preclude the idea of its having been a true in- 

 jected feldspathic dike."* 



In July, 1841, Professor J. D. Dana discovered a series of sandstone dikes 

 at Astoria, near the mouth of the Columbia river, Oregon. f 

 According to Professor Dana — 



" Half a mile above Astoria a sandstone dike five feet wide intersects the bluff from 

 top to bottom, and may be traced following an east by south course across the flat shores 

 to the edge of the river. The rock resembles a half decomposed granite, and seemed 

 at first to be an instance of granite intersecting Tertiary shale. But further examina- 

 tion proved it to be identical with the granitic sandstone of the opposite shores of the 

 Columbia. Large fragments and chips of the adjoining argillaceous beds are imbedded 

 in the sandstone of the dike." 



Four other sandstone dikes were observed, ranging from 5 to 18 inches 

 in width, " and they are generally faulted." Professor Dana remarks : 



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

 sition 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." 



*GeoIogieal Observations on Coral Reefs, Volcanic Islands, and on South America,1851, Part III, p. 

 150. In the same volume, part II, p. 100, Darwin mentions dikes of tuff traversing strata of the 

 same material. 



tU. S. Exploring Expedition, under command of Ch. Wilkes, vol. X, Geology (by J. D. Dana), 

 p. 054. 



(439) 



