224 
THE GEOLOGIST. 
2. Displacement-pseudomorphs ; produced by 
a. Incrustation. 
b. Replacement. 
Magnesite (Mg C) in the form of Calcite (Ca C) is an example of an 
alteratiou-pseudomorph by 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 Jlmr, gypsum, &c., are 
instances of displacement-pseudomorphs, the entire substance of these 
minerals being removed and replaced by the quartz. 
The great importance of pseudomorphs consists in this : that they 
furnish us with a record of changes which have taken place iu 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 filled 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 alcalies, and four oxides given in the list in 
XIII. are the only bases, and the four acids given in the same list are 
the only acids, which at present concern our subject. The acids com- 
bine with the bases to form the (1) Silicates, (2) Carbonates, (3) 
Sulphates, and (4) Borate described ; the comparative importance of 
which in rock-forming minerals is there explained. 
