166 ME. E. B. C0WPER EEED OX THE GEOLOGY OF [May 1S95, 



the differences, in his opinion, being largely brought about by the 

 varying depths at which portions of the original magma conso- 

 lidated. Though I cannot hold that his interpretation applies to 

 the lava-flows in the Fishguard area, yet I have referred to his 

 paper at some length since it contains the first minute account and 

 figures of this type of structure in acid igneous rocks, and recognizes 

 its distinct character. Teall mentions a similar structure in a quartz- 

 felsite from the Cheviots, but not under this name. 1 Iddings de- 

 scribes it iu certain dyke-porphyrites in the Yellowstone Park, 2 and 

 G. H. Williams states that it is well developed in acid lavas on South 

 Mountain, in Southern Pennsylvania and Maryland. 3 Recently 

 referring 4 to these, the latter writer says: — ' It can there be proved 

 in some cases to be of secondary origin, as it occurs in plainly 

 devitrified glasses, and it is the writer's opinion that such enclosing 

 quartz-areas will, in many cases, prove to have originated subse- 

 quent to the solidification of the rock.' The same author here 

 also defines the meaning of the terms ' poikilitic ' and ' micropoiki- 

 litic ' as ' rock-structures, whether primary or secondary, conditioned 

 by comparatively large individuals of one mineral enveloping smaller 

 individuals of other minerals, which have no regular arrangement 

 in respect either to one another or to their host.' 



Eutley describes a Builth rock which under the microscope 

 ' breaks up into a series of irregularly rounded and angular patches 

 when rotated between crossed nicols ' 5 ; and, in the devitrified 

 glassy rocks of the Grlyders and Snowdon, there are mentioned large 

 irregular patches with uniform extinction but badly defined bound- 

 aries under crossed nicols. Hatch 6 , at the last Newcastle meeting 

 of the British Association, in speaking of two Wicklow soda-felsites, 

 mentions a ' patchy devitrification ' in one case, and in the other 

 ' a tendency of the groundmass to depolarize in patches.' 



Teall, on p. 308 of his ' British Petrography,' speaks of ' aggre- 

 gates of double-refracting grains or patches, often of extreme irregu- 

 larity, but of considerable size,' occurring in some felsites and 

 rendered visible only with crossed nicols. 



The above-mentioned authors all apparently refer to this same 

 structure — now conveniently designated ' micropoikilitic' In the 

 Pishguard rocks it constitutes a blurred or a definite mosaic of 

 quartz-patches, pervading the whole rock or merely portions of it, 

 or even only parts of a microscope-slide. The patches, which 

 behave as single crystal-individuals, may form a connected mosaic 

 of coarse or fine grain ; or they may occur more or less isolated from 

 each other, with a groundmass of the cryptocrystalline or microlitic 

 type intervening between them. Each patch may be definitely marked 

 off from its neighbours, and in this case the mosaic is generally 



1 ' British Petrography,' 1888, p. 343. 



2 Twelfth Annual Report U.S. G-eol. Surv. 1891, p. 589. 



3 Amer. Journ. Sci. ser. 3, vol. xliv. (1892) p. 482. 



4 'Journal of Geology,' Chicago, vol. i. (1893) p. 179. 



5 ' Felsitic Lavas of England and Wales,' Mem. Geol. Surv. (1885) p. 21. 



6 Rep. Brit. Assoc. (1889) p. 568, and Geol. Mag. 1889, p. 547. 



