54 OUTI.IM 9 OF I [ELD-GEOLOGY PART i 



and mud. Taking them in tliis NVK the geolo- 



onsiders, with regard to those he encount* 

 the field, whether tlu \ ^mental, Derivative, or 



Stratified (6*A/J//<r), and, if so, whether they are conglomer- 

 ates, sandstones, shales, < In :<>nes, ironstones, or 

 other varieties of this ;i< : \\hether, on the 

 hand, they are Crystalline or Igneous rocks, and if so, 

 \vhethertheyshould Declassed as granite, syenite, diorite, 

 andesite, l)asalt, gabbro, serpentine, or other species of 

 this family ; or whether they are to be called Foliated or 

 Metamorphic rocks, such as gneiss, micaschist, or 

 blende-slate. To be able to answer these questions, the 

 observer must have trained his eye by the examination of 

 good typical specimens of rocks. This is a kind of know- 

 ledge not to be obtained from books ; it can only be 

 gathered from patient and intelligent handling of the 

 rocks themselves. In the field, the observer who ha 

 this training in PETROGRAPHY, as the study of rocks is 

 termed, can recognise most of the rocks he encounters. A 

 pocket-knife, lens, and acid-bottle will assist him if his 

 eye does not readily detect the characters of the stone. 

 But it will often happen that he requires to subject a 

 rock to more careful examination at home, before he can 

 decide as to its nature and name ; while, in other cases, 

 he may require to call in the aid of an experienced 

 petrographer. 



It is absolutely necessary that the field -geologist 

 should familiarise his eye with certain important minerals 

 which enter largely into the composition of rocks, so as 

 to be able to identify and distinguish them, and thereby 

 the rocks which they constitute. For this purpose he 



