THE ORIGIN AND DESCENT OF ROCKS. 



435 



build up the original forms of the crystals from which the sand grains 

 were derived (Fig. 354). Sometimes a film of iron oxide has formed 

 about the grain of sand before the addition of the new sihca. This, or 

 some difference of color, may clearly distinguish the original grain from 

 subsequent additions. Sometimes the adjacent grains of sandstone are 

 rebuilt in this way until the interstices are completely filled. When this 

 has been accomplished, the sandstone becomes a quartzite (Fig. 355). 



Fig. 356. — Feldspar crystals enlarged by secondary growth. Magnified 50 diameters. 

 A A = original grains ; 55 = enlargements ; D = unfilled spaces. (Van Hise.) 



Most quartzites indeed appear to have been formed in this way, but 

 mainly under special conditions that promote the deposition of sihca. 

 Grains of other minerals, such as feldspar, are subject to similar 

 secondary enlargement (Fig. 356). 



Sometimes the new material is deposited in the form of concentric 

 shells about the particles of sediment, building them up into Httle spheres. 

 Rock formed of such spherules is known as oolite, from the resemblance 

 of the grains to the roe of fish (Fig. 357). Sometimes the nuclei of the 

 concretions are grains of quartz sand, and the added concentric 

 layers are of calcium carbonate. In this case the structure is quite 

 obvious; but perhaps more frequently the nuclei are minute and 

 difficult to identify, and the concentric shells make up the main 

 mass of the grains. Certain formations, as the oolitic limestone of 

 Indiana and elsewhere, and the Upper and Lower Oolites of England, 



