Rev. J. F. Blake on the Mont an System. 439 



times the strike is parallel to that of the foliation-planes ; the two 

 modes of occurrence are occasionally observable in different portions 

 of the course of the same dyke, e. g. in one traversing that part of 

 the Man-of-war group known as the Spire. This dyke is also 

 noticeable from the fact that it appears to be traversed by veins of 



The dykes vary in width from 18 inches to several feet. 

 In his notes on the specimens Mr. Teall says that the rocks may 

 be arranged in four groups : — 



1 , principally occurring in the outer islands, are of the Men Hyr 

 type, consisting of felspar, quartz, dark mica, and hornblende ; the 

 quartz and felspar sometimes exhibit relations characteristic of 

 igneous rocks, at other times they form a fine-grained granulitic 

 aggregate, the latter being probably the result of dynamic meta- 

 morphism. This granulation is carried to a greater extent in some 

 of the islands, as in Taylor's Rocks. The rocks possess the minera- 

 logical composition of quartz diorite, and may be termed tonalite- 

 gneisses ; they may originally have been eruptive tonalites. 



2, occurring chiefly in the inner islands, are of the nature of 

 granulitic gneisses and granulites, confining the latter term to rocks 

 in which the quartz and felspar are present wholly in the form of a 

 micro-crystalline mosaic of fairly uniform grain. In some of these 

 foliation is not well marked. Such rocks occur in " Wiltshire," &c. 



3. Rocks showing a passage from the granulitic rocks to the 

 mica-schists of the mainland, as the brown schistose rocks of Lab- 

 ham Reefs. The Enoch rock, a coarse quartzless hornblende schist, 

 also has affinities with the mainland schists. 



4. Dykes traversing the gneisses, consisting of porphyritic felspars 

 lying in a ground-mass of hornblende and granulitic felspar. The 

 hornblende is probably secondary after augite, and the rocks epi- 

 diorites. These dykes have been affected by deformation, and 

 sometimes pass into actinolite schists near the junction with the 

 gneissose rocks. 



In conclusion, the period of dynamic metamorphism, of which the 

 most striking results are seen in the schists of the south-western 

 portion of the Lizard peninsula, was posterior to the formation of 

 the basic dykes. There is no evidence of igneous action in this 

 district since the period of metamorphism. 



2. "The Monian System." By the Rev. J. F. Blake, M.A., 



F.G.S. 



The object of the author was to show that the whole of the rocks 

 which, under various names, had been described as Pre-Cambrian in 

 Anglesey constitute a single well-characterized system, of which the 

 various divisions hitherto described are integral and inseparable 

 parts. 



The evidence of these rocks being Pre-Cambrian was first discussed, 

 and it was shown that the greater part of it went no further than 

 to prove them Pre-Ordovician, the basal conglomerates being asso- 

 ciated with rocks of Arenig age, though from the occurrence of these 

 2 G2 



