480 



R. MALLET ON THE MECHANISM OF 



-Intersecting Dykes. 





No. 23, which have been abruptly curved in contrary directions and 

 broken into numerous voussoir-like fragments with but little dis- 

 placement from each other by pressure while gripped by the matrix, 

 as indicated by fig. 3. 



In the dyke No. 7 there is intersection of one dyke with 

 another, the surfaces at the intersec- 

 tion being so altered and dislocated Fig. 4.- 

 that it is impossible to say which of the 

 two is the intruder ; both dykes are 

 here violently distorted and broken, as 

 roughly shown in fig. 4. In many in- 

 stances, as in the dyke No. 5, one or 

 more adjacent dykes have been broken 

 across into short fragments, twisted, 

 curved, or straight, which have been 

 forced away from each other for dis- 

 tances of many feet or yards, and now 

 occupy the most irregular positions in 

 the matrix, as remarkably evidenced in 

 the dyke No. 16, of which the annexed 

 figure 5 is a rough representation. 

 These trauspositions and dislocations 

 are of much interest and importance. 

 They indicate the vast extent as well as the force of the internal 

 movements, due principally to gravity, which are constantly taking 



Fig. 5. — Dykes fractured and displaced. 



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J\i w 







place in the mass of volcanic cones. Material in active volca noes 



