460 SCIENTIFIC RECORD FOR 1883. 



detrital grains of this mineral composing the rock, in such a manner 

 that there is perfect optical and crystalline continuity between these 

 and the deposited layer, each fragment serving as a nucleus and having 

 been changed into a definite crystal. This process, by infiltration, has 

 served to consolidate the grains in many cases into a hard quartzite. 

 The careful microscopic studies of Irving have furnished many illustra- 

 tions of this process in the Potsdam and Saint Peter's sandstones in Wis- 

 consin. The crystalline nature of many sandstones was long ago pointed 

 out by filie <le Beaumont, and later by Daubree. Brainard, of Ohio, in 

 18G0, called attention to the crystalline character of certain sandstones 

 in that State, which he then conceived to be due to chemical deposition 

 from water. Young's later observations are to the same effect, and 

 show that the deposited quartz is oriented with the inclosed grain. 

 Irving notices that the tendency of silica to deposit itself upon a crys- 

 talline nucleus has been exerted on the surface of weathered sand- 

 stones, forming thereon a vitreous crust. A deposition of dissolved 

 silica is also conspicuous in the Potsdam sandstone of Lake Cham- 

 plain, as described by the writer, where certain beds are changed into 

 hard quartzite and others are made up of grains agglutinated by a 

 chalcedonic cement. From similar facts Hall long since concluded that 

 the beds of Potsdam sandstone in Iowa had been in great part depos- 

 ited from aqueous solution. It is probably in the absence of nuclei 

 which determine the crystallization of dissolved silica that this sub- 

 stance often separates in a hydrated uncrystalline form as hyalite, 

 opal, or silicious sinter. 



THE ORIGIN OF IRON ORES. 



The great deposits of magnetite and specular iron are generally held 

 to be of aqueous origin, though some have maintained them to be erup- 

 tive. It is known that magnetite crystallizes out during the slow cool- 

 ing from fusion of basic ferriferous silicates, and is common in many 

 eruptive rocks. Julian has suggested that the separation of magnetite 

 from these, and its concentration by the action of water, as seen in the 

 washing of sands on a beach, may have given rise to the beds of mag- 

 netic iron ore found in crystalline stratified rocks. This view has been 

 criticised by Newberry, who maintains the accepted theory, that they 

 have been deposited from solution. He points out that these beds are 

 often of great thickness, and are frequently directly inclosed in crystal- 

 line limestones, or in highly argillaceous schists, both of which rocks in- 

 dicate a very different mode of deposition to that required for iron sands, 

 which, as concentrated on our sea-shores, are also accompanied by 

 layers of quartz sand. 



PALEOZOIC ROCKS OF NEVADA. 



Arnold Hague, in a preliminary report of the Geological Survey of 

 the United States, has given us the results of a detailed study of the 



