THE MICROSCOPE. 39 



which are so rife, will find the flesh white, delicate, and 

 wholesome. 



Zones of growth are clearly to be seen in the Section. 

 What periods of growth these may indicate is, I believe, yet a 

 moot question. 



TuFFEN West. 



In his Notes, Mr. T. West has observed, that more detailed 

 description of some of the names used in microscopical notes on 

 rocks might be useful to some unacquainted with the subject. I 

 will, therefore, add a little explanation in reference to Felspar, etc. 



The Felspars belong to the Siliceous group of minerals, and 

 are divided into two great groups : — ist, the MonocHnic or 

 Orthoclastic ; and, the TricUnic or Plagioclastic. 



The cleavage-planes of the former form an angle of 90° ; 

 those of the latter, of less than 90°. 



I. — Orthoclase ; composition, K Si + Al Si3, part of the Al 

 (Alumina) may be replaced by Fe (Iron) or Mn (Manganese) and 

 part of the K (Potassium) by Na (Sodium) or Ca (Calcium). 



Its varieties, accordmg to Colour or Lustre, are : — i. — Adu- 

 laria; 2. — Common Felspar (the ordinary Orthoclase of Granite, 

 etc.) ; 3. — Sanidine (only found in true volcanic rocks). 



Orthoclase Felspars are essentially Potash Felspars. 



II. — Plagioclase ; composition Na Si + Al Si3. Ca, K, or 

 Mg, may replace Na. 



I. — Oligoclase. 2. — Labradorite. Some of the Na may be 

 replaced by Ca. The rich play of colours shown by this is due 

 to numerous inclosures, consisting of Microlithic needles and 

 plates, the latter being frequently crystals of Haematite, yellowish 

 by transmitted light, but gorgeous in reflected ; specular iron also 

 occurs. These MicroHthic inclosures, as a rule, follow the 

 cleavage-planes of the crystals, or sometimes they cross them. 

 3. — Saussurite (impure Labradorite). 4. — Anorthite. 



Opalescent Felspar, or Labradorite will be found extremely 

 beautiful if viewed either as an opaque object, care being taken 

 to rotate it under the light, so as to get the most brilliant 

 effects of colours ; or else as a polariscope object, when the 

 polysynthetic (twin) structure of the Plagioclastic Felspars will 

 be seen. 



The Plagioclastic Felspars may almost invariably be known 

 under the microscope by their parallel striping, which in the 

 polariscope presents a very beautiful appearance. It is owing to 

 the parallel growth, side by side, of numerous crystals. 



