112 



LITHOLOGICAL GEOLOGY 



exposure to the air. The structure may he simply- 

 cracked irregularly, as in many volcanic dikes. But 

 frequently there are transverse fractures, 

 producing a columnar structure, so that 

 a dike is like a pile of columns. For a 

 short distance from the walls, the struc- 

 ture is generally imperfect (Fig. 130) ; 

 and in many cases there is an earthy 

 layer along the sides, or even a laminated 

 structure parallel with the walls (Fig. 

 131), produced by the friction of the rising liquid mass against the 

 walls of the fissure. 



Dikes are sometimes metalliferous; and, when so, the ore is com- 

 monly found near the walls, and often penetrates also the enclosing- 

 rock. Some of the richest mines of the world are connected with 

 dikes, or with igneous ejections. 



Veins never have the transverse columnar structure of dikes. The 

 simplest consist of one kind of material, — as quartz, granyte, heavy 

 sj^ar, — and are alike from side to side. But others have a banded 

 structure not found in dikes, consisting in an arrangement of the- 

 material parallel to the walls. Fig. 132 represents such a vein con- 

 sisting of ten bands : 1, 3, and 6 are bands of quartz ; 2 and 4, of a. 

 gneissoid granyte ; and 5, of gneiss. Of banded veins, the simplest 

 is a vein with three bands, one central ; but the number may be a 

 score or more. The bands may be partly metallic ores of different 

 kinds, and calcite, barite, fiuor spar, may make the alternating bands,. 



Fig. 132. 

 6 & 4321 2 4 5 6 



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iii 



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!vii 



instead of granyte or gneiss. In Fig. 133, there are three sets of 

 bands : an inner, C, and two outer. The left-hand one consists of 

 four bands or combs, a, b, b, c of earthy minerals, with ore along the 

 centre ; and that to the right of two combs, a, a, with a central line 

 of ore ; while C is a simple band, and it may be of ore. A great. 



