OUTLINES OF FIELD-GEOLOGY 



PART I 



Again, a rock may at one place be so compact and tough 

 as to be broken with difficulty, though at a short dista ; 

 may be as soft as loose gravel or sand. Wide variations in 

 texture likewise occur ; a mass of rock will heri 

 coarsely crystalline or almost granitoid aspect, while there it 

 may be so close-grained as to app* homogeneous. 



In his field-work, therefore, the learner will discover 

 by experience what are the essential characters in 

 case. Reserving more precise and detailed investigation 

 for indoor work (see Chap. XVI), he will find that with 

 the unaided eye and such instruments as can be carried 

 in the field, he can take note of the following particulars 

 of the rocks: i. Fresh fracture and weathered surface. 

 2. Apparent structure and texture. 3. Hardness and streak. 

 4. Colour. 5. Smell. 6. Feel. 7. Behaviour in mass. 

 1. Fresh Fracture and Weathered Surface. 

 All rocks yield more or less to the corroding action of 



the atmosphere. Some, like- 

 pure limestone, are dissolved 

 by rain, the dissolved material 

 is carried away, and the sur- 

 face of the rock may remain 

 bare, clean, and hard. But 

 most of them show a more or 

 less distinct crust or outer 

 crumbling skin, which is 



thicker or thinner, according to the resisting power of the 

 rock on the one hand, and the vigour of the decomposing 

 agents on the other. In this outer weathered crust we 

 may often observe the composition of the rock better than 

 on the fresh fracture. The very existence of such a crust 



FIG. 5. Weathered crust, showing 

 concentric zones of oxidation. 



