THE FELSPAKS. 
217 
THE FELSPAES.- 
BY T. H. WALLER, B.A., B.SC. LOND., PRESIDENT OF THE SOCIETY. 
Among the minerals of which the so-called igneous or crystalline 
rocks are made up 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 crystalline 
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-an-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 to fall back on what 
we may call a Linnaean 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 us 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 one plane of 
symmetry. If we take a rectangular block of wood and place it on a 
looking-glass, the reflected image will appear simply a continuation of 
the block, and the same would be the case whichever of the faces of 
the block was placed on the glass. There are, therefore, three planes 
* Read before the Birmingham Natural History and Microscopical Society. 
+ From Lamorna, near Penzance. The granite contains very large white 
crystals of Felspar, 
