117 



live apparent strata formed of these substances are, 

 in fact, veins. If this rule be true, the nature of 

 the rock is the easiest criterion, and' will always 

 be sufficient for the purposes of distinction. 



It must be recollected, that veins of quartz and 

 of calcareous spar, are excepted from the general x 

 rules that apply to rock veins. They form how- 

 ever a distinct class of mineral veins, highly inte- 

 resting to the geologist from their frequency. 



They are seldom of very large size, although 

 quartz veins occasionally attain a considerable di- 

 mension. They are seldom tabular, or continu- 

 ously laminar for a long space ; but are more 

 generally ramose, and more or less largely fila- 

 mentous. They intersect each other frequently, 

 and are often, at the same time, shifted. 



There are some rare instances in which strata, 

 and that most frequently when they are in contact 

 with the overlying rocks, are minutely divided, 

 extenuated, and contorted, so as to put on the ap- 

 pearance of veins, on a superficial observation. 

 Such false veins even present occasional appear- 

 ances of ramification. They are most frequent in 

 the argillaceous schists ; but they also occur in mica- 



