36 ROCKS OF THE CHANNEL ISLANDS. 



ness, and the glitter of the particles of mica have probably 

 given rise to the statement found in the old treatises on 

 the Channel Islands that emery was one of the products of 

 these islands. One of these dykes, which occurs near the 

 mouth of St. Sampson's Harbour, has been mistaken for a 

 sandstone, although it occurs as a nearly vertical dyke. 

 These mica traps are referred by Rev. Hill to the Kersantite 

 group, which are assigned by Dr. Barrois to the close of the 

 Carboniferous period. A few other dykes also cut the Jersey 

 conglomerate; these are stated by M. Noury to be dioritic, 

 although their state of disintegration renders it difficult to 

 determine what their composition is. A large vein at Mannez, 

 in Alderney, cuts the grits, which is considered by Rev. Hill 

 to be a fine-grained diabase, but is styled an andesite by 

 M. Bigot. Whether these last mentioned dykes are more 

 or less recent than the mica traps, I see no evidence to 

 determine. 



If the above facts and inferences are accepted, it would 

 appear that the gneiss, the hornblende schists, the chlorite 

 schist, the diorite, the granites of Alderney, Herm and Jethou, 

 and some of the Guernsey granites, also a large proportion of 

 the dark blue veins, and, perhaps, some of the Guernsey and 

 Alderney pink felsites, are Archaean — the Jersey Argillites, the 

 Jersey and Cobo granites, the diabase dykes, the Jersey quartz 

 felsites and rhyolites, and probably the quartz felsites of 

 Alderney and Guernsey, the Jersey conglomerate and the 

 Alderney grits are comprised within the limits of the 

 Cambrian system, and the mica trap dykes belong to the 

 Carboniferous period. With the exception of some superficial 

 pleistocene deposits, no newer rocks are found in the Channel 

 Islands. 



P.S. — Since the above paper was read, a paragraph has 

 appeared in the Revue Scientifique of October 18th, 1890, 

 stating that M. de Lapparent on a recent visit to Jersey has 

 determined that the quartz felsites or rhyolites belong to the 

 Cambrian period, and not to the Permian as he had concluded 

 on a former visit, and that these rocks are traversed by 

 granulitic veins. Possibly, therefore, the granulite mentioned 

 as cutting the granite may be of a later date also than the 

 quartz felsites. A note of a communication on the same 

 subject by M. De Lapparent to the (English) Geological 

 Society in Nature, November 27th, 1890, says that M. De 

 Lapparent assigns the conglomerate, which clearly overlies 

 the quartz felsites to the very base of the Silurian formation. 



