124 Maw — Cambrian Rocks of Llanberis. 



layers not only differ as to the state of mechanical subdivision of 

 their constituents, but, notwithstanding the sudden and numerous 

 alternations which often occur within a few inches, have a very 

 different composition, and must have been derived from distinct 

 sources of material. The most striking difference is in the pro- 

 portion of the magnesia, of which the green layers contain seven 

 times the amount of that in the slate, and the slate contains more 

 than six times the amount of alkalies found in the green beds. 

 Again, lime, magnesia, and protoxide of iron are much more largely 

 present in the green beds than the slate, and a comparison of the 

 two analyses, as well as the microscopic structure, conclusively show 

 that the green bands could not have been derived by chemical 

 segregation from the slate. ^ Moreover, the thicker bands contain 

 mechanical fragments of a pale green slate. In the Glyn quarries 

 (Plate VII.) they invariably range with the stratification, and appear 

 to have been subject to all the movements and alterations the slate 

 has undergone since its deposition ; the adjacent slate, generally 

 on the under side only, but sometimes on both the upper and under 

 side of the band, has been changed to a pale green (the lightest 

 portions of Plate VII.), and the isolated spherical blotches are of the 

 same character, surrounding a small nucleus of the green matter. 



As I have referred to the character of this pale discolouration in 

 another paper recently communicated to the Geological Society,^ I 

 would here only observe that analyses of the bleached bands fail 

 to support the view expressed by Mr. Sorby^ that ''they are con- 

 cretions of a peculiar kind formed round bodies lying in the plane 

 of bedding." Their general composition is identical with that of 

 the blue slate in which they occur, differing only in the reduced 

 amount of peroxide, with no increase of protoxide of iron present. 

 Mr. Sorby has pointed out that these discoloured blotches have been 

 much elongated and distorted in harmony with the lines of cleavage 

 (Plate VII., Fig. 2), indicating that the discolouration was antecedent 

 to the cleavage. The banding and bleaching has also been inter- 

 rupted (as in Plate VII., Fig. 1) by the dislocations of the slate. 



A strong proof of the contemporaneous interstratification of the 

 dark green layers is that their upper surface, which is comparatively 

 level, graduates into the overlying slate, as though in the deposition 

 of the succeeding layers of mud, some of the green matter had been 

 washed up and intermixed with it. The junction with the slate on 

 the under side is, however, defined with remarkable precision, and 

 instead of being nearly level it has a curiously undulating outline, 

 pockets of the green rock here and there running down into the 

 slate below. It seems difficult to explain vihj the green beds should 

 thus graduate upwards into the slate, whilst the slate beds shew no 

 gradation on their upper surface into the overlying green layer. 



Another remarkable feature is that they are not horizontally con- 



^ The view suggested by Trofessor Phillips at the late meeting of the British 

 Association. rf. 



^ "On the Disposition of Iron in Variegated Strata." 

 3 " On Origin of Slaty Cleavage," Edinboro' New Philosophical Journal, July, 1853 



