Outlines of Geology, 83 



exhaling an earthy odour when breathed upon. It is nearly in- 

 fusible when pure ; but, as has already been stated, it enters 

 into combination with the other earths, so as to produce fusible 

 compounds. When in a very comminuted state, it dissolves in 

 the caustic fixed alkalies, and in most of the acids ; — with potassa 

 and sulphuric acid, it forms the characteristic octoedral crystals 

 of common alum. Alumina has so strong an attraction for water 

 as to retain it for some time, even at a red heat. In its pure 

 and crystalline form, it constitutes the gem called sapphire, of 

 which there are several coloured varieties. 



Magnesia, like lime, forms a part of several compound mine*f 

 rals, most of which have a greenish colour, and a soapy feel. 

 Mixed in the state of carbonate, with carbonate of lime, it is 

 found in a subspecies of limestone, to which it imparts peculiar 

 characters. It is insoluble in the alkaline solutions, but readily 

 soluble in many of the acids ; and its presence is announced by 

 a precipitate of carbonate of magnesia, on adding carbonated 

 fixed alkali to its solution ; while, on the other hand, bi-carbonate 

 of ammonia, which throws down lime, does not precipitate mag- 

 nesia unless phosphate of soda be added : and upon this is found- 

 ed an elegant method of detecting the presence of magnesia, de- 

 vised by Dr. Wollaston. Now the different sands, clays, marls, 

 and limestones of the upper strata, are composed of mixtures of 

 the substances just enumerated, and they constitute, in chemical 

 combination, and ^vith a few ununportant additions, the principal 

 components of the primary rocks. 



The regular succession of the Earth's strata has been already 

 partially referred to, as determined by the inquiries of some of 

 the earlier geologists ; and as the order of this succession, and 

 the respective characters of the series, will occupy much of our 

 attention, it may be right to explain it by a few preliminary 

 observations. Let us suppose a traveller, for instance, depart-, 

 ing from London and travelling westward towards North Wales ; 

 and let us direct his attention to the ground which he passes 

 over — he first traverses a tract of clay and sand, tlien he enters 



G2 



