206 



we have to note that those which have been examined fall into three 

 groups : 



I. The dykes of Cleveland and Acklington. 

 II. The dykes of Hebburn, Tynemouth, Seaton, and Hartley. 

 III. The dykes of Hett and High Green. 



Those of the first group are of Tertiary age ; those of the second 

 group are of unknown age ; those of the third group resemble the Whin 

 Sill in composition and specific gravity, and are, in all probability 

 of pre-Tertiary age. 



ANALYSES OF NORTH-OF-ENQLAND DYKES. 



99-14 98-56 99'76 10002 



99-99 99-82 100-76 99-64 



, Gr, 2-8.8 ,., 2-95 ,,, 2-06 ,,, 2-02 



