224 



THE GEOLOGIST. 



2. Displacement-pseudomorphs ; produced by 



a. Incrustation. 



b. Replacement. 



Magnesite (Mg G) in the form of Calcite (Ca 0) is an example of an 

 alteration-pseudomorph hj exchange. The Ca of the Calcite is gra- 

 dually removed and replaced by Mg ; thus converting the substance 

 of the crystal into Magnesite, while it still retains the form of Calcite. 

 The example given of quartz in the form of fluor, gyiysum, kc, are 

 instances of displacement-pseudomorphs, the entire substance of these 

 minerals being removed and replaced by the quartz. 



The gi'eat importance of pseudomorphs consists in this : that they 

 furnish us with a record of changes which have taken place in the 

 rock-world, of which without their help we should have remained 

 entirely ignorant. If in a certain district we meet with a great series 

 of veins jSUed with quartz, a considerable quantity of this quartz 

 having the form of calcite, we have at once revealed to us the impor- 

 tant fact that the veins were once filled with carbonate of lime, 

 which had been removed and replaced by quartz. If the removal and 

 substitution had taken place under circumstances favourable to the 

 development of the quartz in its own proper form, we should have 

 found no pseudomorphs, and have remained entirely ignorant of the 

 change that had taken place. And this is probably more frequently 

 the case in nature, — the form alters with the alteration of the sub- 

 stance ; but when this is not so, and the form remains while the 

 substance is altered^ that pseudomorphous form becomes an important 

 geological monument. 



XXIII. General Chemical Relations of Mineral Salts. — 

 When two binary compounds unite together to form a higher one, 

 chemists call the electro-positive of the uniting compounds the hase^ 

 and the electro-negative the acid. The higher compound made by the 

 union of this base and acid is called a salt. 



The four earths, three alcaiies, and four oxides given in the list m 

 XIII. are the -only bases, and the four acids given in the same list are 

 the only acids, wdiich at present concern our subject. The acids com- 

 bine with the bases to form the (1) Silicates, (2) CarbonafeSy (3) 

 Sulphates, and (4) Borate described ; the comparative importance of 

 which in rock-foi'ming minerals is there explained. 



