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THE POPULAR SCIENCE MONTHLY. 



microscopically examined. When the granite rocks become decom- 

 posed, as they often do in Cornwall and elsewhere, through the wear 

 and tear of the weather, we frequently find the disintegrated mate- 

 rials so separated that the silicate of alumina of the feldspar forms 

 thick deposits of the beautiful white clay known as kaolin, and which 

 is so valuable to the china-manufacturer. 



The mica of granite is usually a variety called Muscovite, or potash 

 mica; this again on chemical analysis is found to contain, as did the 

 feldspar, silica, alumina, and potash, and also often some iron and man- 

 ganese. There are several different sorts of mica, also, sometimes 

 found in granite, especially Biotite, the composition of which varies 

 from the above ; but all the micas may be known by their being found 

 in flatfish crystals, which may be split up into an infinity of thin leaf- 

 lets. Thus far our unaided eyesight and the help of the chemist have 



Fig. 2. Obthoclase Feldspar. Fig. 3. Plagioclase Feldspar. 



shown us what granite is made of; but we are now beoinnina: to learn 

 that, would we know something of the real history of a rock, a far 

 minuter examination is needful, and geologists are rapidly learning 

 that they must turn to the microscope if they would receive answers 

 to many important questions, both as to the history and also as to the 

 composition of rocks. A marvelous light has been shed during the 

 past few years on rock-structure through this minute investigation, 

 especially with the aid of polarized light. The intricacies of the 

 closest-grained rocks have been disentangled, their component parts 

 distinguished from each other, and the very order and history of their 

 combination in the mass revealed. Now, when we examine our gran- 

 ite beneath the microscope, which can be done by having thin slices 

 prepared, w T e shall learn something about it which we could hardly 

 hope to have discovered without this aid. There has been much spec- 

 ulation as to the origin of granite, whether it is a plutonic that is, 

 an old volcanic rock or whether it is only a deposit from water con- 

 solidated and altered during the lapse of long ages by heat and press- 

 ure : the microscope will help us to the truth. When magnified and 

 examined with the polariscope, a thin section of granite is a very 

 beautiful object, and its different constituent parts stand revealed 

 with the greatest distinctness : we at once learn to see the crystals of 



