34 ROCKS OF THE CHANNEL ISLANDS. 



absent or only accidentally present, it is not applicable to the 

 rock now under consideration. This granite like other similar 

 rocks varies in colour, composition, and grain, but is generally 

 of a pink colour due to the orthoclase felspar, which is 

 abundant, and contains both mica and hornblende, though 

 occasionally these minerals are almost absent. This granite 

 is cut by numerous veins of a finer-grained granite or grann- 

 lite, which may be of somewhat later date, or may consist 

 simply of fluid off-shoots from the main mass penetrating the 

 already solidified portions. There are also numerous intrusive 

 dykes of greenish rock which M. Noury calls diabase. In 

 Guernsey, the Cobo granite resembles in a general way the 

 Jersey granite above referred to, and I think may be of the 

 same age. It is intrusive both in the gneiss and diorite, and 

 is certainly much more recent than these rocks, as the very 

 numerous dykes of dark blue fine-grained rock Avhich pene- 

 trate them are cut off by the Cobo granite. This granite is 

 indeed remarkably free from intrusive dykes, the few that are 

 found in it being chiefly, if not all, mica-traps. 



The next rock in order of age is again mostly typically 

 found in Jersey. I allude to the quartz felsites or rhyolites. 

 They vary very considerably in appearance, indicating dif- 

 ference in the original mode of formation or subsequent 

 alteration. In some places they consist of porphyritic and 

 euritic rocks, in other places they present a remarkable banded 

 structure, doubtless due to the flowing of the rock when in a 

 molten condition. Intermixed with the banded rhyolites are 

 breccias, and large masses of altered Argillite. These latter 

 rocks are especially conspicuous at Havre Giffard, in Trinity 

 Parish. These structures are indicative of a volcanic origin, 

 but the whole of the rocks appear to have undergone con- 

 siderable subsequent alteration. Possibly the more compact 

 and crystalline portions may represent the eruption of the 

 rock through the Argillites, while the banded rocks and 

 breccias may be due to the flowing of the molten rock on the 

 surface, carrying along with it fragments and masses of the 

 disrupted Argillites and of the already solidified portions of 

 the erupted rock itself. These quartz felsites have attracted 

 considerable attention from the spherulites occurring in many 

 of the bands, some of them of considerable size. This appears 

 to confirm the view that these rocks were formed at the 

 surface. Dykes of quartz felsite are abundant in Guernsey, 

 and still more so in Alderney, but rare or absent in Sark. 

 These dykes are intrusive in the Archaean rocks, and may be 

 connected with the Jersey rock above described, or they may 



