230 REPORT— 1887. 



Report of the Committee, consisting of Professor T. Gr. Bonney, 

 Mr. J. J. H. Teall, and Professor J. F. Blake, appointed td 

 undertake the Microscopical Examination of the Older Rocks of 

 Anglesey. {Drawn up by Professor J. F. Blake, Secretary.) 



The Secretary of the Committee reports that it has been thought desirable 

 for the adequate exaniination of the questions which arise in connec- 

 tion with tlie crystalline schists and associated rocks of Anglesey to have 

 a large number of sections — about 300 — cut from specimens from various 

 localities. The cutting and preparation of these have occupied so much 

 of the year as not to have left adequate time for the detailed study they 

 require. 



A map is exhibited showing the localities from which the rocks from 

 which slices have been prepared have been obtained. These are in 

 nearly every part of the island where the older rocks occur, and certainly 

 incluile examples of every important variety. For stratigraphical pur- 

 poses, to show the distribution of the vai-ions types, which cannot be 

 with certainty distinguished in the field, a still larger series would be 

 desirable ; but for general questions connected with the origin of these 

 rocks the collection is probably sufficient. 



These preliminary results obtained by the first examination will be 

 liable to modification and correction when more time has been given to 

 their study ; but the following points seem fairly well established at 

 present : — 



1. The quartz rocks have two distinct origins ; one group consists of 

 ordinary beds of quartz sand which have been more or less compacted 

 and foliated by the development of some chloritic or other mineral in the 

 interstices, and the other group has the original quartz grains irregularly 

 scattered and imbedded in quartz which has been developed in the rock 

 itself, somewhat after the manner of the quartz in a vein. 



2. Passages may be traced from true chloritic schists, in which the 

 largest original saud-graius only are left here and there, into breccias, in 

 which the matrix has not yet been crystallised to its full extent, but 

 which remains in a dusty or granular state. 



3. Tiie presence of this green mineral, geneially called chlorite, is 

 characteristic of certain parts of tlie whde series of Anglesey rocks, 

 whether taken from the newer or the older portions, though its amount 

 and definiteness vary to a great extent. 



4. This same chloritic mineral is characteristically combined with 

 quartz in what one might almost call a micropegmatitic manner, except 

 that the mineral is rather in rounded blebs, arranged in a botryoidal 

 manner. 



5. The less crystalline or dusty members of the series are often 

 divided by narrow opaque lines of the finest dust running more or less 

 parallel, but interosculating and undoubtedly produced since the first 

 formation of the rock. The more crystalline the rock the more rare is it 

 to find such lines in them. 



6. The granitic and dioritio rocks, which are found associated with the 

 schists or ashy rocks, more generally with the latter, are distinguished 

 by the presence of accessory minerals, such as zircon, sphene, rutile, and 

 apatite. 



