GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



367 



So far as they have yet been examined, it would ap- 

 pear that they occur under the following circumstances. 



1. They are found only at or near the junction of the 

 granite and clay slate. 



2. They are not metalliferous. 



3. They have no general direction or position ; some 

 are quite vertical j others nearly so, others at different 

 angles and some quite horizontal. 



4. Their direction is usually as straight, and their size 

 as regular as those of true veins ; but in some cases they 

 become smaller as their distance from the granite mass 

 increases. 



5. Their greatest length has never been ascertained ; 

 some have been traced as far as 200 feet. 



6. The granite of the veins generally appears different 

 from that of the main body ; it is of much smaller grain ; 

 it contains a much larger proportion of quartz and very 

 little mica ; sometimes, indeed, scarcely any perceptible 

 mica. ' 



7. The slate which is contiguous to the veins becomes 

 almost imperceptibly changed from clay slate to mica 

 slate, and sometimes has even the appearance of gneiss. 



8. The slate v/hich is close to the veins is frequently 

 much harder than that which is more distant from them, 

 and its texture, in general, not so slaty. 



9. The veins have been traced to the granite mass, 

 with which they appear to be in complete union, and to 

 form one body, losing their character entirely as to veins. 

 Whether the other veins unite with the granite mass or 

 not, has not been ascertained, as the point of junction is 

 seldom accessible or even visible. 



10. One of the veins has been traced from the slate 

 into the granitic mass. 



11. Some veins are closely connected with the slate, 

 and the two bodies appear intimately united and insepa- 

 rable ; in fact they appear contemporaneous. Others, 



