18 PICTURESQUE SKETCHES 



account of the rocks which have been called "Primi- 

 tive" but shall quit this subject with a remark which, 

 from the manner in which I have spoken of granite, 

 may perhaps be necessary. It is this : that there are 

 many kinds of granite and granitic rock, some of them 

 very different from ordinary granite in appearance, 

 and that there are also many other so-called primitive 

 rocks very different in structure ; but these varieties 

 do not prevent the account I have given from being 

 sufficiently accurate for my purpose, and I trust the 

 reader will not suppose, when he has read through 

 this little volume, that he has learnt everything in 

 Geology. 



Next in order to the crystalline rocks, and almost 

 as widely extended, there are two or three others, often 

 themselves crystalline, but bearing evident marks of 

 what is called " mechanical structure," or, in other 

 words, of having been deposited from water. These 

 may be described as gneiss, mica-schist, and clay-slate.""" 

 If we imagine common granite coarsely pounded, and 

 thrown into a vessel of water, it will arrange itself 

 at the bottom of the vessel in a condition very 

 much like that of gneiss, which is indeed nothing 

 else than stratified granite. If the water in which 

 the pounded rock is thrown is moving along at a 

 slow rate, and that part of the granite called felspar 

 happens to be somewhat decomposed, as it often 



* Under this name " clay-slate," I only mean here to include those 

 slates, whether of distinctly crystalline structure or not, which present no 

 marks of having contained fossils. That there are such, no Geologist will, 

 I suppose, doubt ; but when the name clay-slate is given, as it is some- 

 times, to fossiliferous beds, they ought to be referred at once to the period 

 indicated by the kind of fossils discovered. 



