44 PEOF. T. G. BONNET AND THE REV. E. HILL : [Peb. I9I2, 



Y. The Basic Dtees. 



Basic dykes, which the older geologists would have called * green- 

 stones/ are abundant in the Channel Islands. Mr. Hill in 1884 

 gave a general description of those in Guernsey/ including, 

 however, among them one or two intrusive masses, which, on 

 closer examination, seem more nearly related to the diorites ; and 

 he mentioned in 1889 an interesting example in Alderney.^ 

 During our late visit we paid some attention to those in Guernsey, 

 but found them to be so numerous and in many cases so difficult 

 of access that we soon gave up the idea of collecting materials for 

 a complete study, which could only be done by a residence of some 

 months in the island. We therefore contented ourselves with 

 examining a few of the more representative varieties. 



We take first one that, strictly speaking, ought not to be included. 

 It is from a dyke, about 4 feet wide, cutting the gneiss of L'Eree 

 Point, opposite to Lihou Island. In the field it resembled a some- 

 what decomposed diorite, rather less coarse-grained than the 

 normal 'speckled' variety of that rock; but the microscope shows 

 porphyritic crystals of hornblende, felspar, and quartz, scattered 

 m a microcrystalline ground-mass. The first mineral is variable 

 in size, sometimes fairly idiomorphic, sometimes composite, the 

 grains averaging about '01 inch in diameter, with a somewhat 

 ' fiaky ' structure and extinguishing at angles rather uniformly low. 

 The second is a plagioclase, the extinction-angles suggesting a 

 somewhat basic oligoclase. The third (quartz) is not common, and 

 is much corroded by the ground-mass. A few grains of iron-oxide 

 are also present. The ground-mass is a mosaic of clear minerals, 

 probably to a large extent felspar (untwinned and perhaps reconsti- 

 tuted), with occasional interstitial belonites of hornblende and 

 flakelets of a very pale brown tint — possibly another form of the 

 same mineral. One or two cracks are largely cemented by epidote. 

 So this rock is very possibly more nearly related to the diorites 

 than to the material of the ordinary basic dykes. 



The next specimen is also rather abnormal. It cuts gneiss, 

 cropping up irregularly on the rocky shore to the north of the 

 causeway leading to Lihou Island, seldom so much as 6 feet thick, 

 and often less than 3. It contains numerous crystals of hornblende 

 fairly idiomorphic, sometimes nearly an inch long, in a rather 

 compact speckled greenish matrix. In one place it is distinctly 

 cut by a fairly thick dyke of diabase (with small phenocrystals of 

 felspar) which runs up into the headland ; in one or two more it 

 appears to be cut by and welded to a rather compact diabase, but 

 in others it has a chilled edge in which the porphyritic hornblendes 

 become smaller and fewer until they disappear (which suggests 

 that these may have been, at least to some extent, formed in situ). 

 The larger hornblendes, under the microscope, prove to be only 



1 Q. J. G. S. Yol. xl (1884) pp. 416-18. 



2 Ibid. vol. xlv (1889) p. 384. Those of Sark are briefly noticed in vol. xliii 

 (1887) p. 332, and again by the present authors in vol. xlviii (1892) p. 140. 



