662 GEOLOGY. 



granite in veins and beds among these strata, they have been invaded from below by 

 other igneous rocks, the porphyries, greenstones, and basalts, which are found intermingled 

 with them, forming dykes and disrupting masses peering up above them. Examples of 

 this are of common occurrence in the Hebrides, and several parts of the Highlands. 

 Reference has been made to the conversion of coal into coke in the neighbourhood of 

 a trap dyke ametamorphic change and analogously, the gneiss and mica-schist strata, 

 subjected to a high temperature by the invasion and interjection of heated masses forced 

 up from below, have had much of their original character defaced, and have become 

 in part crystalline, without entirely obliterating those traces of laminar and stratified 

 arrangement which proclaim their origin. In those instances, where such traces are 

 obscure in the gneiss and mica-slate, and they exhibit a pseudo-crystalline texture, 

 presenting such a striking resemblance to granite as to render their identification difficult, 

 the reason is, that besides being composed of granitic detritus, they have experienced 

 metamorphic change, by the action of heat, through contact with masses protruded among 

 them in a state of igneous fusion. In a similar manner, by the application of heat and 

 pressure, the chemist may convert sandstone into quartz-rock, and carbonate of lime into 

 crystallised limestone. From the contiguity of primary strata to granite, and the 

 extensive prevalence of other igneous rocks by which they have been invaded, it is 

 obvious that the absence of organic remains in them is not positive proof of a desolate 

 world being contemporaneous with their deposition. It is certainly possible that the 

 same cause which has rendered them metamorphic may have destroyed fossils imbedded 

 in them, with all traces of their presence, so that we are not in circumstances to 

 say, upon arriving at the era of extant remains, that here commences the chain of 

 organic life, and that the globe, in antecedent ages, was a habitation without an 

 inhabitant. 



Clay Slate System. This is the lower greywacke, a German miner's term for grey 

 rock, also called the clay-slate or Cambrian and Cumbrian group. The latter denomina 

 tion is that of Professor Sedgwick, from Cambria, the ancient name of Wales, and the 

 county of Cumberland, localities where the formation indicated is largely developed. 

 There is a grand general distinction in mineral composition between the two former 

 systems noticed, which are eminently siliceous compounds, and the present, which is as 

 eminently argillaceous, consisting of clays under different degrees of induration, having 

 a fissile structure. It varies, however, from the finest clay-slates to conglomerates, which 

 are fragments of quartz, felspar, and mica, united by an argillaceous cement. These 

 conglomerates, especially in the upper part of the system, are interstratified with slates. 

 Observing the order of superposition, Professor Phillips divides and arranges the system 

 in the following manner : 



UPPER OB CAMBRIAN GROUP. 



Plyntymmon Rocks. Hard, fine, sandy, or coarse grauwacke, and grauwacke-slate, 

 without organic remains, which is locally productive of roofing slates, and generally 

 traversed by an extraordinary abundance of symmetrical fissures. Wales, Cumberland, 

 and Westmoreland, the Laminermuir Mountains, Donegal Range, borders of Dartmoor, 

 North Devon, Charnwood Forest. Thickness, in the Cumberland tract, 3000 feet at least. 



Bala Limestone. Dark, slaty, calcareous rock, variously associated with the slate, 

 and locally rich in organic remains, both in Wales and Westmoreland. Limestone of 

 Ilfracombe and North Devon ; of the Hartz, Norway, Brittany, &c. Thickness, in West 

 moreland, 100 feet. 



Snowdon Rocks. In this division are rocks of various colours, green, blue, purple, 

 &c., and fine or coarse grain. Good roofing slate abounds, and the peculiar fissility, called 



