148 



GEOLOGY. 



veins have often penetrated through the overlying depos- 

 its, and flowed over the rocks which they have displaced, 

 as shown in Fig. 78. Sometimes one vein is displaced 

 or faulted by another, as repre- 

 sented in Fisr. 79. Here is nn 





Fig. T8. 



outline of the section of a boulder found by Prof. Hitch- 

 cock in West Hampten, Mass. It is . a fragment of a 

 granite rock which was traversed by three granitic veins. 

 These, as seen in the boulder, are bed, ef, and gh. The 

 vein bed was made first. The other two were made 

 afterward, displacing or faulting the first, as you see in 

 the figure. As the three veins were made at three dif- 

 ferent times, they are three different varieties of granite. 

 Here, then, is a record of four different formations of 

 granite, the rock itself and the three veins, and how long 

 the intervals were between them we know not they 

 may have been ages. 



Veins differ from dikes in various respects. They are 

 apt to be irregular in shape, larger at one point than at 

 another, while dikes are regular and uniform. Veins 

 are often compound, containing a variety of materials, 

 sometimes a considerable variety, while dikes are sim- 

 ple, containing only one kind of material, as some volcan- 

 ic mineral or trap. And when a vein has only one kind 

 of material, this never has the arrangement that a dike 

 often presents viz., a columnar arrangement, as if one 

 block were placed above another from the bottom to the 

 top of the dike. Those veins which are compound have 

 a very different arrangement from the columnar one of 



