CHAPTER VII. 



THE PHYSICAL AND MINERAL HISTORY OF STRATIFIED ROCKS. 



ALTHOUGH the greater changes which have taken place in the history 

 of the earth are only to be discovered by following the several beds of 

 rock through the country, and observing their relations to each other, 

 and alterations in mineral character and in fossils, yet much may be 

 learned concerning the conditions under which each deposit was accu- 

 mulated, and sometimes even the direction and district from which the 

 deposited material was derived, by minute and even microscopic ex- 

 amination of the particles of which a stratum is built up. This kind 

 of research has been rendered possible by the labours of Dr. Sorby l 

 and Mr. John Arthur Phillips, 2 and it is on the basis of their researches 

 that we give an indication of the ways in which sands, clays, and lime- 

 stones may be made to yield evidence of their history and origin. 



Sand. 



By sand we understand the materials constituting the fine-grained 

 silicious rocks called sandstones. This sand has in every case been 

 derived from the destruction of igneous or metamorphic rocks, and in 

 some cases of chert or flints. The quartz from granite consists of 

 separate grains which often have an irregular and complex form, but 

 the quartz from felsite is much more truly crystalline, and the planes of 

 the crystals are frequently perfect though the angles are more rounded 

 than in the quartz from granite. Sometimes the grains are corroded as 

 though partly dissolved by the action of the alkalies liberated when the 

 associated felspar was decomposed. The quartz derived from gneiss 

 and mica-schist, especially when those rocks have a thin foliation, is 

 remarkable for being flattened in the plane of foliation, and consists of 

 numerous small crystals dovetailed together, so that when broken up 

 it gives rise to a fine-grained sand, or a sand containing grains which 

 show a compound structure ; and if the parent rock contained mica, 

 thin plates of mica are found between the parallel grains of quartz. 

 When the grains are observed under the microscope they often show 

 fluid cavities, frequently with bubbles. This character is conclusive 



1 H. C. Sorby, Address Quart. Jour. Geol. Soc., vol. xxxvi. 



2 J. A. Phillips, Quart. Jour. Geol. Soc., vol. xxxvii. p. 6. 



