Cole — Contact-Phenomena at Junction of Lias and Dolerite. 65 



paralleled by the diabase dykes of Electric Peak, described by 

 Iddings.^ This author refers us with justice to Charles Darwin's- 

 discussion of differentiation in igneous masses through the growth of 

 crystals in a magma of less density. Darwin's view that crystals 

 would in many cases gather towards the bottom of horizontal flows 

 led him, sixty years ago, to one of those philosophic conclusions that 

 have placed him among the greatest and most far-seeing of geological 

 observers. 



Finally, in view of Mr. Jennings's specimen from one of the veins 

 of Portrush, with its fine-grained and coarser zones rich in soda- 

 pyroxene, there is clearly room for further research in this well 

 visited and attractive field. A specimen in the Portlock Collection, 

 probably from Portrush and not from Fair Head, shows a zone of 

 soda-pyroxene and plagioclase, forming a rock of dioritic composition, 

 succeeded by a zone of granular dolerite of finer gTain, this being 

 succeeded, along an interlocked edge, by a zone of hornblende- 

 plagioclase rock, such as one generally associates with the epidioritc 

 phase. But the hornblende in this case cannot be derived from the 

 pyroxene of the dolerite that is seen in the adjacent zone. Are 

 these zones due to successive intrusion, or to marginal differentia- 

 tion, or to contact-alteration ? Moreover, is the somewhat startling 

 epidiorite or aphanite a stranger brought up solid from the underlying 

 schistose series ? 



The main dolerite of Portrush shows a felspathic facies in places, 

 in which the felspar is andesine, as determined by Mr. T. Crook and 

 myself. There are thus possibilities of modification in this mass also, 

 on the one hand by marginal differentiation, and on the other by 

 absorption of material met with in its passage from below. 



^ " The eruptive rocks of Electric Peak and Sepulchre Mountain, Yellowstone 

 Park," Twelfth Ann. Eep. U. S. Geol. Survey, Part i. (1891), pp. 584-5. 

 - Observations on Volcanic Islands (1844), Minerva Edition, pp. 243-5. 



