20 PROF. T. G. BONNET ON THE HORNBLENDIC AND OTHER 



I trust some day to treat the subject more fully ; but I take this 

 opportunity of calling attention to it, because I believe that in it 

 we find a clue which may ultimately enable us to solve many diffi- 

 culties in that most perplexing inquiry — the relation of the meta- 

 morphic and of the igneous rocks. 



IV. Age of the Metamorphic Series. 



A few words may be said in conclusion as to the probable age 

 of this metamorphic series. In my former paper I called it 

 " altered Devonian," following the authority of others which I had 

 then no reason for questioning. In this one I have adduced the 

 reasons which have convinced me that it must be much more an- 

 cient. An immense interval of time separates its rocks from the 

 slaty series found in juxtaposition. Even these deposits are not 

 impossibly older than the Devonian series. According to Mr. J. H. 

 Collins they are, at latest, Lower Silurian*, and their general 

 appearance would very well accord with such an age. We need 

 not hesitate then to assign the metamorphic series to the Archaean 

 Period ; and, so far as we can trust the evidence of mineral con- 

 dition, we may refer them to no very late epoch in this. The 

 amount of alteration in the Lizard sedimentary rocks is indeed less 

 than we find in the more typical representatives of the Hebridean 

 series of Scotland or of the Malvernian of England. It is only in 

 the granitoid bands of the Upper Group (with one exception) that 

 we are reminded of the Dimetian of St. Davids, or the most crystal- 

 line gneisses and " granitoidites " of Anglesey. But all these occur 

 in thick masses, while those of Cornwall are in thin bands. In 

 distinctness of bedding and in general mineral condition we may 

 find parallels among some of the so-called Newer Gneisses in the 

 Central Highlands of Scotland ; but, so far as I know, the latter the 

 resemblances are not very close ; and in touching these we intrude 

 upon a veritable " thistle-bed " of controversy, from which we 

 would gladly keep clear. The closest resemblance known to me is 

 between many members of the Micaceous Group and parts of a 

 micaceous or chloritic series which is well developed in Anglesey, 

 especially in Holyhead Island, on the adjacent coast, and on the 

 northern border of the eastern part of the Menai Straits ; so that, 

 without pressing the parallel too far, we may, I think, provisionally 

 regard the Lizard rocks and these Anglesey schists as belonging to 

 about the same age. The latter, I am aware, have been claimed by 

 one author as altered Bala, an hypothesis which a petrologist can 

 hardly believe to have been advanced seriously. By the Geological 

 Survey they are regarded as altered Cambrian, a view which I think 

 recent investigation has shown to be untenable. A third authority, 

 while claiming them as Archaean, suggests that they are Pebidian. 



* See Trans. Roy. Geol. Soc. Cornwall, vol. x. pp. 5, 47, and Journ. R. I. Cornw. 

 vol. vii. p. 18. He appears, however, to me to have completely mistaken the 

 relation of the hornblende schist and serpentine one to another and to these 

 rocks, and not to add any thing to the work of De la Beche, which he criticizes. 



