Vol. 68.] PETROLOGICAL NOTES ON GUEKNSEY, ETC. 37 



felspar, fairly idiomorphic, is not well preserved, but apparently 

 corresponds with that seen in the other specimen ; so does the 

 quartz, which sometimes shows slight strain-shadows. 



This rock was intrusive in a micaceous diorite of fairly normal 

 character, and probably is identical with the lighter (intrusive) 

 rock in a junction -specimen mentioned in Mr. Hill's paper of 

 1884.^ 



General Conclusions regarding the Diorites. 



The foregoing description shows these diorites to be the products 

 of a magma, which was subject to differentiation more or less local; 

 the one extreme being represented by the very basic form at Fort 

 Albert, the other either by the coarser felspathic intrusions, or 

 by the tonalites in the Grand Havre district. The diorites of 

 Guernsey, as was noticed by Mr. Parkinson, contain more quartz 

 and biotite towards the north or north-west of the island, the 

 microscope sometimes showing the presence of the former material 

 where the aspect of the rock did not lead one to suspect it. 



Where the magma has undergone a marked differentiation, the 

 more basic portion seems to have been the earliest extruded ; and, 

 the later the date is, the more acid is the material. Tbat also 

 holds good in Alderney, where at least three forms occur, one of 

 them sometimes very basic. But is it not possible that some of the 

 granites may also be residual products of the same magma ? The 

 one at Bibette Head, Alderney (intrusive in diorite), is distinctly 

 hornblendic, and in it also the biotite often seems to have been 

 formed at the expense of the other mineral. Besides this, it is 

 not very rich in quartz, and contains a fair amount of plagioclase. 

 Both the ' granites ' of Sark, as described in 1892,^ are hornblendic, 

 and the southern one is really a tonalite. 



The granites of Herm and Jethou are distinctly hornblendic,^ and 

 so are those of Guernsey, especially that of Lancresse (intrusive 

 in diorite). It is, therefore, possible that these also may be yet 

 more acid terms in a differentiation series. 



III. The Acid Dykes. 



These are often of a light brick-red, inclining sometimes to a 

 pinkish red, sometimes to a buff colour. Occasionally they may 

 attain a thickness of 12 or 15 feet, but are more often less than 

 3 feet thick, and may even ' dwindle ' to less than an inch. In 

 texture, they vary from compact to finely granular (the more 

 common) ; in composition, from aplites to microgranites : a ferro- 

 magnesian constituent being very seldom conspicuous, and their 

 edges rarely showing any appreciable chilling. Usually they are 



^ Q. J. G. S. vol. xl, p. 415. The slice, however, shows less biotite. and 

 perhaps no hornblende, but only a small piece of it represents this rock, the 

 remainder belonging to the other, which is a normal diorite (? traces of augite) 

 with a little biotite. 



2 Ibid. vol. xlviii (1892) p. 130. 



3 Ibid. vol. xliii (1887) p. 333. 



