CLASSIFICATION OF BOCKS. 39 



consolidated into rock, in some cases to be again denuded. 

 The rocks now found are classified in various ways by geolo- 

 gists. One convenient method is based upon the process by 

 which they were formed. We thus get three principal 

 classes : 



1. Igneous rocks, subdivided into (i.) Plutonic. 



(ii.) Volcanic. 



2. Sedimentary rocks, subdivided into (i.) Mechanically pre- 



cipitated. 



(ii.) Chemically pre- 

 cipitated. 



3. Metamorphic rocks. 



These terms are almost self-explanatory. Class 1, generally 

 hard and silicious, often crystalline, comprises those rocks 

 which have been formed by solidification from a fused state ; 

 class 2, those formed from the fragments of previous rocks by 

 deposition, generally under water; class 3, rocks (generally of 

 class 2) which have been altered in character since their 

 deposition by high temperature and pressure. 



Another method of classification is based upon structure. 

 Thus we have : 



A. Crystalline rocks, e.g., granite. 



B. Vitreous rocks, c.y., obsidian. 



C. Colloidal rocks, e.g., silicious sinter, nodules. 



D. Fragmentary rocks, e.g., sandstones, shales. 



Rocks of class A and B are generally igneous in origin 

 (exception certain limestones, rock salt, gypsum, &c., which 

 may be classed as crystalline). Class C have probably been 

 deposited from solution. Class D are sedimentary in origin. 



Class 1 include the oldest rocks, from which probably the 

 other rocks have been formed. Rocks of this class generally 

 contain several minerals. 



Minerals are characterised chemically by possessing a per- 

 fectly definite molecular structure and yet showing great 

 variation in composition. Th:s is due to the power which 

 similar isomorphous elements exhibit of replacing each other 

 in a compound without altering its crystalline form or general 



