ON A HILLSIDE IN DONEGAL 351 



example of the intrusion of a magma from which crystals have 

 already separated, such as that observed by Teall ' in the glassy 

 infillings of vesicles in certain lavas. The lava has almost 

 solidified, gases have blown up hollows in it, and yet the 

 residual magma is still fluid enough to run in and occupy 

 the cavities. Another pleasing instance occurs on the coast of 

 Co. Down, 2 where a fine-grained granite, already rich in crystals 

 of quartz and orthoclase felspar, larger than those that 

 developed in the groundwork of the rock, has invaded a dyke 

 of basalt. Blocks of the basalt have been carried off by 

 the granite, and have melted within the invader. In turn, 

 they have reacted on and absorbed material from the granite 

 magma, but have been unable to completely destroy the larger 

 crystals that had already developed in it. In consequence, they 

 now include red orthoclase and quartz, in a ground the constitu- 

 tion of which is not far different from that of the original basalt. 

 Harker 3 has confirmed this observation during his researches 

 in the Isle of Skye, where acid felspars and quartz grains have 

 been transferred from a granite into fragments of a basalt into 

 which it has intruded. 



Salomon, 4 moreover, notes the transference of great crystals 

 of biotite and triclinic felspar from an enveloping quartz-diorite 

 to included fragments of a schist. 



At Green Point, again, near Cape Town, there is an exposure 

 of more than ordinary interest, since it was described by Charles 

 Darwin, 5 the author of the term "foliation," in 1844. The shaly 

 Malmesbury Beds, the oldest recognised rocks in Cape Colony, 

 here come up almost vertically on the shore ; granite, with large 

 crystals of orthoclase, occupies the country under Table Moun- 

 tain to the north, and penetrates the shales at Green Point in 

 a series of parallel sheets. Indeed, as Darwin wrote, films of 

 altered clay slate are " isolated, as if floating, in the coarsely 

 crystallised granite ; but, although completely detached, they all 

 retain traces of the uniform north-west and south-east cleavage." 



1 "On the Amygdaloids of the Tynemouth Dyke," Geol. Mag., 1889, p. 482. 

 3 Cole, " On derived crystals in basaltic andesite, Co. Down," Set. Tra?is. 

 R. Dublin Soc, vol. v. (1894), p. 239. 



3 " Igneous Rocks of Skye," Mem. Geol. Survey of United Kingdom (1904) 

 p. 219. 



4 " Geologische Studien am Monte Aviolo," Zeitsch. d. deutsch. geol. Gesell., 

 Bd. xlii. (1890), pp. 476 and 493. 



5 Geological Observations on Volcanic Islands, Minerva ed. p. 264 



