No. 6.] HARRINGTON — NOTES ON DYKES. 323 



of Tertiary age. They are highly crystalline and do not appear 

 to contain any glassy base. As yet no olivine has been observed 

 either in them or the diabases, but very few sections have been 

 examined, and possibly it will be found on further study. In 

 No. I, a mineral has been observed with the characters of sanidin, 

 and no doubt other minerals will yet be detected. 



The order in which the different minerals have solidified is a 

 matter of interest, apparently not being that of the fusibilities 

 of the constituent minerals before the blowpipe. In the diabase 

 and dolerite it is evident that the apatite has been the first to 

 solidify ; the plagioclase appears to have come next, then the 

 magnetite, and last of all the augite. Mr. J. Clifton Ward 

 gives an interesting example of the apparent order in which 

 the minerals constituting a leucitic basalt near Naples have 

 solidified, which may be noticed in this connection. The 

 minerals are leucite, magnetite, magnesia-mica, feldspar and 

 augite. Of these five minerals the only infusible one is the 

 leucite, and yet Mr. Ward thinks that the last four " were held 

 in solution by leucite in a state of fusion ; and that instead of 

 this mineral crystallising out first, it deposited in succession the 

 magnetite, the mica, the feldspar and the augite, and last of all 

 probably solidified quickly, enclosing within its crystals glass- 

 and stone-cavities, and magnetite and feldspar crystals." 



It is evident that No. V is a very different rock from any of 

 the others described. In some respects it resembles the so-called 

 melaphyres, but contains much more mica than is found in any 

 of which I have seen descriptions. No. VI is as already stated 

 a diorite and needs no further remark here. 



The slight amount of alteration exhibited by some of the 

 ancient dolerites in the Grenville region would no doubt be sur- 

 prising to some, but is not so much, to be wondered at when we 

 consider that they occur in highly crystalline rocks, which would 

 serve to a great extent to protect them from the agencies which 

 have brought about decomposition in dykes cutting the unaltered 

 strata of some more recent formations. 



* Quart. Jour. Geol. Soc. 1875, p. 396. 



