GNEISS, MICA SCHIST, AND SLATE SYSTEMS. 



663 



cleavage, on which it depends, is very general in all the mass of rocks, which are some 

 thousand feet thick, both in "Wales and Cumberland. In both districts also the sedi 

 mentary rocks are much intermixed with porphyry and greenstone, both in seeming 

 beds and dykes, and many parts of the slaty rocks themselves are really amygdaloidal, or 

 else composed of fragments of porphyry and other igneous rocks. No organic remains 

 have been found in the Cumbrian district, but they occur in Snowdon. Thickness, in 

 Cumberland, 6000 feet at least. 



LOWER OR CUMBRIAN GROUP. 



Clay-slate. This is a singular uniform, mass of laminated argillaceous rock, of a 

 dark colour and smooth texture, with vertical cleavage and symmetrical joints. It is 

 devoid of organic remains. 



Chiastolite-slate differs hardly at all from the preceding, except by including crystals 

 of chiastolite and hornblende. 



Hornblende, slate. This is very different from the rock so named in Glen Tilt, for 

 its basis is clay-slate, with intermixed crystals of hornblende or actinolite. 



These three divisions may be about 3000 feet thick in Cumberland. 



Passing through the series ascendingly, there is little occasion for remark in relation to 

 the lower division. The actinolite is only a variety of the hornblende, which forms tho 

 slate called after it, a mineral which occasionally presents fine radiating fibres, and has 

 hence been named after its resemblance to the sun's rays. The chiastolite imbedded in 

 clay-slate, and comprising one of its varieties, is so called from the form of an X, in dark 

 lines, visible on the summits of the crystals. The chiastolite-slate, a soft, dark kind, 

 occurs near Bareges in the Pyrenees ; at St. Jago di Compostella in Spain, and in the 

 Sierra Morena ; at Agnavanagh in Wicklow ; and constitutes in connection with other 

 varieties the great .mass of Saddleback, 2787 feet high, and of Skiddaw, 3022 feet, in 

 Cumberland. 



Snowdonia, a range of variously coloured and indurated argillaceous slates, the lower 

 division of the Cambrian group, is of peculiar interest to the geologist ; for here -\v.e meet 



Skiddaw Mountain. 



