OF UNSTRATIFIED ROCKS AND VEINS. 143 



very important nature. In one of these, already men- 

 tioned, the vein has intersected an alternating series 

 of argillaceous schist and quartz rock ; breaking the 

 latter in a decided manner, but bending the former, 

 which consequently, bears the same marks of flexure 

 that are seen where granite traverses gneiss. In the 

 others, there is a striking difference in the forms of 

 the veins where the fissure has been transverse to the 

 laminae of the invaded schist and where it has con- 

 formed to these ; the vein having the usual parallel 

 form of trap in the former case, and resembling the 

 irregular veins of granite in the latter. 



As the veins of trap are most frequently of large 

 dimensions, so they are, generally, either vertical or 

 inclined at high angles to the horizon. Thus probably 

 they indicate, by both circumstances, the great depths 

 at which the parent masses lie, and the nature and 

 place of the force which produced the fissures. But 

 that they frequently also occupy very low angles, will 

 immediately be shown, when the positions of veins 

 with respect to the strata are considered. 



Although the term fracture implies that the strata 

 are disunited in some direction across their laminar 

 arrangement, it does not follow that all veins hold 

 courses thus intersecting these. While they may be 

 broken at any degree of obliquity, they are sometimes 

 also separated according to the planes of their strati- 

 fication, or according to that of their laminar struc- 

 ture. In the latter case, the veins are rarely large, 

 and are more or less parallel to the laminae of the in- 

 vaded rock ; in the former, strata, either similar or 

 dissimilar, are sometimes completely separated, even 

 for considerable spaces, so as to permit a large and 

 parallel mass of the intruding rock to occupy the 



