variations in color; but, unlike quartz, 

 shows regular cleavage faces. Feld- 

 spar is always crystalline, but good 

 crystals are not common. It is very 

 difficultly soluble, yet readily yields to 

 the influence of weathering. A feld- 

 spathic rock hence readily crumbles. 

 During the process of disintegration, 

 the feldspar may change from a clear, 

 hard, glassy mineral to a dull, opaque 

 substance. This product of disintegra- 

 tion is our common white clay. With 

 quartz, then, feldspar is of great im- 

 portance in the forming of soil. 



Allied to the feldspar group of 

 minerals as regards cleavage, and yet 

 of far different special characteristics 

 is the class of substances known as 

 mica. How many of us ever think of 

 'the so-called isinglass of our stove 

 doors as a mineral substance? Yet 

 transparent mica, muscovite, is the 

 source of that household convenience. 

 A study of the specimens of mica in 

 our stove door will provide abundant 

 ideas of the nature of mica. We have 

 often noticed how, under the influence 

 of excessive heat, the isinglass splits 

 into thin sheets, thus showing the 

 cleavage of the mineral. These plates 

 of mica are of especial value in giving 



cleavage to rocks which would other- 

 wise fracture irregularly. The cleavage 

 of slates and of the common shale 

 rocks is due to the presence of mica 

 particles which have, at some period in 

 the history of the earth's crust, through 

 the action of heat and pressure, been 

 arranged along definite planes. Isin- 

 glass represents the transparent variety 

 of mica. Other varieties are brown 

 and even black, owing to the presence 

 of traces of potassium, magnesium, 

 iron, etc., in varying degree. Some 

 micas do not easily decay, and so we 

 frequently see glittering particles 

 among the fine grains of soil and the 

 sands of beaches. 



The minerals already mentioned, 

 quartz, feldspar, and mica, are the com- 

 ponents of a large part of our granites. 

 In the case of the red Scotch granite, 

 another silicate, hornblende, replaces 

 the mica. Various silicates of economic 

 value are asbestos, a variety of horn- 

 blende, and augite, which are silicates 

 of magnesium and iron or calcium; 

 and talc, which is a silicate of magne- 

 sium containing water. A great number 

 of gems are found among the silicates, 

 including tourmaline, garnet, topaz, 

 beryl, and chrysolite. 



THE DANGER FROM THE IMPORTATION OF ANIMALS. 



ftN abstract of J. S. Palmer's essa\' 

 on "The Danger of Introducing 

 Noxious Animals and Birds" 

 appears in Our Animal Friends. 

 There are several societies in this coun- 

 try for the express purpose of purchas- 

 ing and importing European birds. 

 One society in Cincinnati has con- 

 tributed Sq.ooo to this object, and other 

 cities have raised considerable sums. 

 Our contemporary thinks it would be 

 well that all such experiments should 

 be made under the sanction of govern- 

 ment experts of the Department or 

 Agriculture. In addition to voluntary 

 importations, it often happens that 

 animals are unintentionally brought 

 into the country, as trading-vessels 

 have carried the European house 

 mouse all over the globe, and the intro- 

 duction of rabbits into Australia is 

 perhaps the most striking example of 

 the dangers of unconsidered importa- 



tions. They were introduced for pur- 

 poses of sport, and were liberated near 

 Melbourne in 1864. Within twelve 

 years they had spread over the country 

 and become a veritable plague, and 

 millions of dollars have been spent for 

 bounties, poisons, and other methods 

 of destruction. Thousands of miles of 

 rabbit-proof fences have been built, 

 and in 1887 no less than 19,182,539 rab- 

 bits were destroyed in New South 

 Wales alone, and the rabbits seem to 

 be on the increase. The little Indian 

 mongoose was imported into Jamaica 

 to cope with a plague of rats and 

 proved most effective, but after it had 

 destroyed the rats it turned its atten- 

 tion to the domestic animals and poul- 

 try, so that the islanders would now be 

 glad if they could get rid of the pests. 

 Such are a few examples of the danger 

 of disturbing nature's balance. 



