THE FELSPARS. 



THE FELSPAKS. 



BY T. H. WALLER, K.A., 15. SC, LOND., PRESIDENT OF THE SOCIETY. 



Haul hrf'ore tlic Suricti/, Mki/ 22nd, 18S3. 



Among the minerals of which the so-called igneous or crystalline 

 rocks are made np there appear certain which are distinguished from 

 the others by their hardness, colour, and specific gravity, and which 

 go by the general name of the Felspars. I have, however, found it 

 impossible to formulate a definition which would include them all 

 without giving it such wide limits as to destroy its value. Thus there 

 are silicates of alumina and potash, or of alumina and lime, or of 

 alumina and soda, or of alumina with any two of the other oxides. 

 The ratios of the silica to the bases vary greatly, and the crN'stalline 

 form is either monoclinic or triclinic, and the specific gravity varies 

 between 2-57 and 2'75. 



I have, therefore, ventured to risk the charge of unsystematic 

 procedure, hoping that even if we cannot exactly express in words 

 the definition of the whole group, the differences between the various 

 members of it, and some of the more striking characteristics, may 

 profitably employ half-au-hour this evening, especially as there is, so 

 far as I know, nothing in the case of minerals which answers to the 

 natural system in Botany, and we have therefore tu fall back on what 

 we may call a Linnsean system of description. 



Beginning, then, with a concrete example, and taking a coarse 

 granite,* such as there is on the table before you, we pass over the 

 black mica scales and the transparent glassy quartz grains, and fix 

 our attention upon the opaque white crystalline constituent of which 

 there is so much in the specimen. We observe first of all that some 

 of the fractures are smooth and shining, evidently such as are familiar 

 to i;s as cleavage planes, and examining them a little more closely we 

 find in any one crystal two sets of them meeting in an edge, so that 

 the crystal can easily be broken up into prisms. The goniometer tells 

 us that the angles of these fragments are right angles, and from this 

 circumstance the mineral has received the name of Orthoclase, as 

 cleaving at right angles. If now we can obtain a crystal separate from 

 the rock and examine its shape more accurately, it becomes evident 

 that the crystallographic system to which we must refer it is the 

 monoclinic ; that is, there is only one plane along which we could 

 divide it and have the two parts similar — i.e., only 0)ie plane of 

 symmetry. If we take a rectangular block of wood and ))lace it on a 

 looking-glass, the reflected image will appear simply a continuation of 

 the l)lock, and the same would be the case whichever of the faces of 

 the block was placed on the glass. There are, therefore, three planes 



* From Lamorua, near Penzance. The granite contains very large white 

 crystals of Felspar. 



