Cole — Contact-Phoioincna at Junction of Lias (Did Dolcrito. 57 



and the true basalt, in alternating strata a few inches in thickness, 

 both in the peninsula of Portrush and in the outlying isles, the 

 Skerries. He rightly judged that this interlamination was due to 

 " the injection of one of the rocks into the other." Sir R. Griffith, in 

 an annual address for 1835, which is not published in the Journal of 

 tlie Geological Society of Dublin, added valuable observations ; ^ and 

 Portlock- has given a most valuable literary history of the successive 

 researches on the altered Lias of Portrush. The rock is again 

 described in a memoir published by the Geological Survey of Ireland 

 in 1888.3 



Among the minerals described by Oldham in Portlock' s memoir, is 

 one referred with doubt to brouzite.^ The analysis given is 

 admittedly unserviceable, since the flaky mineral was not separated 

 from the rock. The original specim.ens, which came from the 

 Portrush peninsula, consist of a dark-grey crystalline type of the 

 altered calcareous shale, with red-brown micaceous plates lying at all 

 angles at one surface. These plates, where they meet on one another, 

 produce the effect of being the bounding planes of solid crystals, just as 

 the mica does in the peridotite known as scyelite.^ A mica- 

 lamprophyre sent me by Mr. J. St. J. Phillips, from a dyke at 

 Orlock, Co. Down, shows a similar structure. Another instance 

 is seen in the biotite which has abundantly arisen in an inclusion, 

 probably of sedimentary origin, gathered by me from the granite of 

 Ballymagreehan Quarry, Castlewellan. 



My own observations at Portrush have not enabled me to trace the 

 spot whence Portlock's well developed specimens were obtained; 

 but, from the detection of smaller examples, I have no doubt that the 

 crystalline plates hitherto described as bronzite were found close to a 

 junction with the dolerite. A specimen selected for a thin section 

 shows, indeed, a film of dolerite in contact with the altered shale of 

 which the main mass is composed. 



In the first place, the flaky mineral is undoubtedly a brown mica. 

 It has the characteristic cleavage and pleochroism, and is practically 

 uniaxial in sections parallel to the cleavage. It arises in certain 

 zones of the altered rock, the larger crystals lying, for instance, at 



^ See abstract in Portlock, op. cit., p. 43. 

 2 Ibid., pp. 37-44. 



Explanatory Memoir to sheets 7 and 8, p. 18, 

 * Report on Geol. of Londonderry, &c., p. 742. 



^Judd, "Tertiary and older Peridotites of Scot land." Quart. Juuin. Gtol. Sue. 

 London, vol. xli. (1885), p. 4U-2. 



