VEINLETS IN THE SILURIAN AND DEVONIAN 69 
Where veins pass through chert masses, most of the calcite 
crystals extend from wall to wall, and are oriented with their longer 
dimensional axes parallel rather than transverse to the vein walls— 
a fact that is likewise true of many non-fibrous veins in limestone 
and shale. Since this may be observed in the largest as well as 
the smallest veins, it means that the average number of vein crystals 
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Fic. 5.—Vein consisting of calcite (C), pyrite (P), and quartz (Q), with walls of 
limestone (ZL). The pyrite replaces both the limestone and the vein calcite. 
in contact with unit area of the wall tends to decrease with the 
growth of the vein. In other words, it is believed that with con- 
tinued growth those crystals having any advantage, because of 
greater size, or more favorable orientation or location, tend to 
increase in size partly at the expense of their less fortunate neigh- 
bors. This conclusion is supported by the manner in which the 
inclusions have been displaced in some of the veins. The 
