GG 



THE GEOLOGIST. 



— not more than five. These are — silicon, aluminium, calcium, oxygen, 

 and carbon. The states of combination in which they occur are — silica, that 

 is, silicon combined with oxygen; alumina, or aluminium with oxygen; 

 and lime, or calcium with oxygen ; and this lime is, for the most part, or, 

 at all events, to a very large extent, in combination with carbonic acid, 

 constituting marble and limestone in its various forms. Perhaps next in 

 abundance we may rank magnesium ; then, perhaps, would come hydrogen, 

 iron, sodium, potassium, manganese, chlorine, sulphur, and phosphorus. 

 The hydrogen is that existing in combination with oxygen in the form of 

 water, and present in a state of solid combination in all clay. It is not 

 there, as chemists term it, as hygroscopic water, water simply of moisture, 

 which can be expelled at a low temperature, but it is there in a state of 

 actual solid combination, and may therefore be considered as one of the 

 constituents. 



The geologist everywhere meets with problems of the highest interest 

 which chemistry alone can solve ; yet it is somewhat surprising that in this 

 country the application of chemistry to the solution of geological phe- 

 nomena should hitherto have received so small a share of attention. Not 

 a few persons have attained the reputation of geologists who have either 

 been ignorant of the great foundations of the philosophy of geology, or 

 have had a very slight knowledge of the subject. To know and to remem- 

 ber the order of superposition of rocks, and to be able to recognize the 

 fossils which they respectively contain, does not entitle a man to rank as a 

 philosophical geologist. As well might a taxidermist lay claim to the title 

 of zoologist, or an ornithologist to the title of botanist. The conquests 

 which remain to be achieved in geology will, doubtless, result from the 

 special study and application of the various sciences, and there is assuredly 

 no line of investigation which promises richer fruit than chemistry. 



The subject of this first lecture is silicon, perhaps the most abundant, or, 

 certainly, one of the most abundant elements in the solid crust of the 

 earth. This silicon has only recently been investigated in anything like a 

 satisfactory manner. It exists in, and is the foundation of silica in its 

 various forms. Silica exists in the well-known form of quartz, and con- 

 sists of silicon combined with oxygen. In sand and in all clay, and 

 in igneous rocks of various kinds, it is an essential constituent. In fact, 

 it is everywhere. This silicon, when once united with oxygen, requires an 

 extraordinary amount of affinity, or the exercise of an extraordinary force 

 to detach it therefrom. It exists in three distinct states, the amorphous or 

 formless state, in the graphitic state, and in the state of crystallized octa- 

 hedral silicon. 



In its three states, silicon differs considerably. In the amorphous state 

 it occurs in the form of a chocolate-brown powder. In the graphitoidal 

 state it is exactly like graphite, or very similar, occurring frequently in 

 small hexagonal plates, as produced in the process for making aluminium. 

 Then we have the octahedral form, the same form and the same crystalline 

 system as that to which the diamond belongs. But here it is a most beau- 

 tiful substance, of a metallic lustre, and a dark bluish-grey colour, consider- 

 ably more blue than ordinary graphite, and more metallic in lustre. Silica 

 consists, in round numbers, of about 48 parts of silicon and 52 of oxygen. 

 The atomic formula adopted, and first suggested by Berzelius, is Si 0 3 , 

 representing one equivalent of silicon combined with three of oxygen ; 

 but there are reasons for supposing that the more accurate expression is 

 Si G 2 , one equivalent of silicon combined with two of oxygen. Long ago 

 it was ascertained by Sehafgotsch that silica exists in two very different 

 states. In one state crystallized as quartz, having a specific gravity of 



