754 PRINCIPLES OF STRATIGRAPHY 



by lime cementation so as to form a fairly cohesive rock, showing 

 well the oblique bedding of the fore-set beds. 



The porosity of the rock is an important factor in aiding cemen- 

 tation. Other things being equal, the more porous rocks will have a 

 better chance of cementation. Thus the Columbian gravels (Pleis- 

 tocenic) of the Raritan Bay region in New Jersey are frequently 

 cemented into a hard pebbly rock, the yellowish well-worn quartz 

 pebbles being embedded in a deep brown sandstone cemented by 

 iron oxide, the whole resembling a giant peanut brittle. The under- 

 lying Cretacic strata, on the other hand, are unconsolidated except 

 where locally some of the sands are bound together by iron oxide. 

 Certain layers in the Monument Creek Tertiary sandstone of Colo- 

 rado Springs are strongly cemented by iron oxide, while the re- 

 mainder of the sandstone mass is free from such cement. As a 

 result, monuments are carved by the wind out of these rocks, the 

 iron-cemented layers forming the capping stones of these monu- 

 ments. In the Miocenic deposits of Baden, near Vienna, the shell- 

 bearing pebble beds are often cemented by lime into a fairly re- 

 sistant rock, while the clays are entirely unconsolidated. 



The principal minerals deposited between the particles of a rock 

 to form a cement are lime, iron and silica. Silica may be in the 

 form of a colloidal cement, but in quartz sandstone it is far more 

 often deposited in such a way as to have optical and crystallographic 

 continuity with the silica of the grain it surrounds. This secondary 

 enlargement of a quartz grain, forming more or less perfect crystals 

 which interlock closely, is not an uncommon thing and may result 

 in the formation of a hard and strongly indurated rock. Neverthe- 

 less, such close cohesion of new grown crystals does not always take 

 place, and the mass will fall to pieces at the blow of a hammer, 

 leaving a mass of angular quartz crystals which only under the 

 microscope show that they represent the secondary development by 

 addition of the originally more or less well-rounded quartz grains. 

 This is not an uncommon source of angular quartz grains. 



Van Hise mentions as the most important cementing substances : 

 silica, iron oxide and aluminum oxide, among the oxides ; calcite, 

 dolomite and siderite among the carbonates, both hydrous and 

 anhydrous silicates,* and marcasite and pyrite among the sulphides. 

 (Van Hise-34: (5^/-d2^.) 



The Keweenawan sandstone of Lake Superior may be cited as a 

 case in which the cementation is largely due to the deposition of 



* Among the hydrous silicates are: (i) zeoHtes and prehnite; (2) chlorites; 

 (3) epidotes; (4) serpentine and talc. Among the anhydrous silicates are feld- 

 spars, hornblende and mica. 



