122 



LITHOLOGICAL GEOLOGY. 



Figs. 124, 125, 126. Veins much broken or faulted : in 124, four faults within 

 a length of eighteen inches ; in 125, six faults in six feet ; in 126, the broken 

 parts of the vein of unequal breadth. 



Fig. 128. 



Fig. 129. 



Figs. 127, 128, 129. Other faulted veins, 127 a and b, six feet apart, and still 

 different in their faults ; 128, 129, other interrupted veins. These dissimilarities 

 between the parts of one faulted vein, as in 126, and between the parts of two 

 parallel veins, as in 127, arise from an oblique shove of the parts either at the 

 time of the fracturing in which the veins themselves originated, or at some 

 subsequent fracturing. 



The points here illustrated are, — 



The great irregularities of size in veins along their courses, 

 swelling out and contracting ; their occasional reticulations ; their 

 frequently embracing portions of the enclosed rock ; their nume- 

 rous faultings or breaks and displacements. 



132. (4.) Structure. — Dikes. — Dikes consist essentially of the 

 same kind of material from side to side and at all heights, where not 

 altered by exposure to the air. The structure may be simply 

 massive, or 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 structure is generally imperfect 

 (fig. 130) ; and in many cases there is an 

 earthy layer along the sides, or even a lami- 

 nated structure parallel with the walls |§s^ 

 (fig. 131), produced by the friction of the 

 rising liquid mass against the walls of the 

 fissure. 



133. Veins never have the transverse columnar structure of 

 dikes. The simplest consist of one kind of material, — as quartz, 

 granite, heavy spar, — 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, consisting of eleven bands : 1, 3, and 6 are 



