Prof. Sedgwick on the Structure of large Mineral Masses. 479 



chanicalj and contain organic remains. But in the centre of Skiddaw Forest, 

 in some parts of Cornwall, in Anglesea, and in the south-western coast of 

 Caernarvonshire (as stated in a preceding- note, p. 471), and also in a great 

 portion of the Highland regions of Scotland, we have finely foliated and 

 highly crystalline schists (such as chlorite schist, mica schist, &c.) which con- 

 tain no organic remains, and (in some instances, at least,) are older than the 

 slaty groups above described. In some of these older groups the cleavage and 

 laminations of mica and chlorite are transverse to the beddin"-. This is, I 

 believe, the case in some parts of Skiddaw Forest, and it is certainly the case 

 in some parts of Anglesea, as is proved in the beautiful section of Holyhead 

 Mount, published by Professor Henslow, in the Transactions of the Cam- 

 bridge Philosophical Society*. In general, however, the foliated uneven 

 layers of these older formations belong, I believe, to beds, and not to cleavage 

 planes ; and the oldest and most crystalline rocks, designated by the general 

 name of schists, have no true slaty cleavage^ in the sense in which I have 

 used the term. 



A mechanical rock may appear highly crystalline, because it is composed 

 of crystalline parts, derived from some preexisting crystalline rock. Thus, we 

 have in Scotland masses of sandstone so beautifully recomposed from granite, 

 that it is not always possible to distinguish them from the parent rockf. In 

 the same way, the highly crystalline texture of mica schist 7/20?/, in some cases, 

 originate in causes purely mechanical. I believe, however, that the structure 

 of these old crystalline schists has been greatly changed since their deposi- 

 tion ; but the same thing may be asserted of all slate rocks whatsoever, and 1 

 do not see why the term '^metamorphic" should be applied exclusively to one 

 class of them. 



Again, if we apply the term ''metamorphic" to the old crystalline schists, and 

 at the same time assume that the changed structure has originated in igneous 

 action, we generalize, I think, a great deal too fast. Such structures some- 

 times may, and sometimes may not, have been caused by igneous action ; and 

 it appears to me rash to limit the great modifying powers of crystalline forces 

 to particular ranges of temperature. At all events, the great parallel cleavage 

 planes, and some other changes, above described, appear to me entirely un- 

 like the altered structures produced by igneous rocks on the masses which 

 are near them. But I do not presume to decide either on the exact tempera- 

 ture, or on the time, necessary to the perfect development of cleavage planes. 



Before I conclude this section I cannot help recommending, not a new 



* Vol. i. Plate XV. -|- Geological Transactions, New Series, vol. iii. p. 140. 



3 Q 2 



