154 



The Scottish Naturalist. 



form in which felspar is of most importance, industrially, is in its 

 product of kaolin or china-clay, which is the resultant of the 

 complete decomposition of felspar by the action of carbonic acid 

 and water. 



The next substance found in sand that I shall mention is mica, 

 well-known by its glistening flakes. This also is a familiar con- 

 stituent of granite. It is perhaps better known to us as a con- 

 stituent of certain sandstones, than of sand. Mica is found in 

 Siberia in large flakes ; and has been used instead of window- 

 glass. It is so finely laminated that it permits of being divided 

 into extremely thin plates. Mica, besides being a rock constituent 

 of granite, is abundantly present in Gneiss and Mica-schist, the 

 latter being a foliated rock composed of mica and quartz, in which 

 garnets are commonly imbedded. The origin of mica has been 

 the subject of much interesting speculation. It is composed of 

 the same elements as felspar, with the addition of potash, fluoric 

 acid and magnesia, and has been claimed as the descendant or 

 resultant of felspar and other kindred minerals. 



Sand, besides the minerals above mentioned, contains also in 

 many cases, acicular or thread-like crystals of tourmaline (dark 

 red or silvery), and rutile. Besides these inorganic substances, 

 we find the carbonate of lime largely represented in some sands, 

 generally in the form of fragments of shells and corals. 



ORIGIN OF SAND. 



In considering the question of the origin of sand, we naturally 

 look for information to the sand itself. While there is much that 

 is interesting in the microscopical examination of sand, there is 

 not much in this to guide us in considering its origin. 



Sand, as such, is not found in the deeper portions of the great 

 ocean beds. The formations there are found to be composed of 

 an ooze or clay, in many cases made up of the shells of globigerina 

 or foraminifera. It is near the shores of continents and islands 

 that sand is found. Has it then been washed out of the soil by 

 the action of rivers in ordinary course or in times of flood ? The 

 deposition of extensive sandbanks near the entrance of all large 

 rivers, points unmistakeably in this direction. 



The disintegration of rocks by atmospheric agencies, the 

 abrading force of the waves of the sea, the washing-out of mineral 



