THE ARCH.fcAN RO 51 



his memoir on the "Geology of North Wales"; tin 

 complex of the central area was described by Dr. C. Callawa . 

 1902 ; Sir A. Geikie noted the resemblance between some of the 

 schists and those of Scotland*; the north-western part of the island 

 was mapped by Dr. C. Matley, and the Ordovician tract* separated 

 from the Archaean schists in 1899. 18 Finally, during the last fifteen 

 years the whole island has been re-surveyed by Mr. E. Greenly, 

 whose map and memoir will shortly be published by the Geological 

 Surveys. Meantime I am indebted to Mr. Greenly for the follow- 

 ing condensed account of the pre-Cainbrian rocks. 



Though no fossiliferous Cambrian rocks have been found in the 

 island the relation of the schistose complex to the Ordovician, and 

 of the Ordovician to Cambrian on the mainland, leaves no doubt of 

 the pre-Cambrian age of that complex. 



Rocks of Archaean age reach the surface in five principal areas 

 and in several smaller inlying tracts. They consist for the most 

 part of greenish schistose grits, alternating with gritty phyllite* 

 and tine chloride mica-schists ; there are also other mica-schist*, 

 about the origin of which some difference of opinion still exist*. 

 There are, moreover, many bands of quartzite and of limestone, 

 some being of considerable thickness. 



Across the central part of the island, ranging south-west and 

 north-east, is an axis-like tract of highly crystalline rocks, including 

 various types of basic and acid rocks which have a deep-seated 

 aspect, i.e. gneisses, hornfelses, diorite, and granite. The relations of 

 this group to the sedimentary series have been much obscured by 

 subsequent movements, but some of the granites are certainly 

 intrusive into the adjacent rocks which are now in the condition 

 of crystalline hornfels. 



Interbedded with the sedimentary rocks in many parts of the 

 island are bands of basic igneous rock. Most of these are now 

 chloritic and hornblendic schists, with (locally) large masses of 

 glaucophane schist. Some of the chloritic schists were probably 

 volcanic tutfs, but here and there, especially in the south-west near 

 Newborough, the original structure is beautifully preserved, and the 

 lavas are of that pillowy or ellipsoidal form and variolitic type 

 with which radiolarian cherts are so often associated. 



No unaltered cherts have been found, but nodules and lump? of red 

 jasper occur abundantly in these lavas, filling the interspaces between 

 the ellipsoids. Similar jasper also occurs in the form of nodules and 

 thin beds in many of the limestones, and seams of fine jaspery purple 

 phyllite are also of frequent occurrence. From this mode of occur- 

 rence these jaspers have been regarded by some writers as altered radio- 

 larian cherts, but their radiolarian origin has not yet been proved. 



