512 Forbes — The Microscope in Geology. 



The terms primary and secondary are here used quite indepen- 

 dently of geological chronology. Primary rocks (of all ages) might 

 be called "ingenite or subnate rocks" {i.e. such as are born, bred, or 

 created within or below), ^ whilst the term " derivate rocks" would 

 be appropriate for the latter, since directly or indirectly they are all 

 derived from the destruction of the former. 



I. Pktmakt or Eruptive Eocks. 



This class includes rocks which have made their appearance in 

 many, if not in all epochs, from the most ancient to the most recent, 

 from the old granitic outbursts to the eruptions of the now active 

 volcanoes ; and if, as is now generally admitted, the earth be re- 

 garded as having been once a molten sphere, the consolidated origi- 

 nal crust of the globe would pertain to this class of rocks. 



Mineralogicallj'' they consist of crystallised silicates, with or 

 without free quartz, and usually containing many other minerals 

 in minor quantities, especially metallic compounds, as magnetite, 

 titanoferrite, iron pyrites, etc, which last are frequently present in 

 so minute a quantity as only to be detected by the microscope. 



Whatever be their geological age, or from whatever part of the 

 earth's surface they be taken, the microscopical inspection of such 

 rocks shows immediately that they possess certain general and 

 definite structural characters, distinguishing them at once from all 

 other rocks. 



The mineral constituents of such rocks are seen to be developed as 

 more or less perfect crystals, at all angles to one another, thereby 

 indicating that the entire mass must have been one time in a state of 

 liquidity or solution (aqueous or igneous), sufficient to allow of that 

 freedom of motion absolutely essential to such an arrangement of 

 the particles.^ 



The microscopic examination already made of many hundred 

 sections of eruptive rocks, differing widely in geological age and 

 geographical distribution, shows that in all rocks of this class, 

 whether of the most compact, hard, and homogeneous appearance, or 

 occurring in the softest and finest powder, like the ashes and dust 

 frequently thrown out by volcanoes ; a similar crystallised arrange- 

 ment and structure is present and common to them all. Lavas, 



1 These rocks are indiscriminately called Tolcanic, igneous, plutonic, crystalline, 

 etc. The term crystalline, although characteristic of these rocks, is not exclusively 

 so, and is consequently less appropriate ; many normal sedimentary beds, as rocksalt, 

 gypsum, etc., are perfectly crystalline, and others when altered by metamorphic 

 action, become more or less so. 



2 Experiments show that analogous structure can be produced by at least three 

 different methods, all of which, however, agree in the necessity of the mass being in 

 a state of complete liquefaction previous to crystallisation ; from — 



1. Their solutions in water or other menstrua. 



2. Aqueous fusion or melting of hydrated bodies in their water of crystallisation. 



3. Igneous or hydro-igneous fusion. 



Crystalline structure may nevertheless develop itself by a molecular movement in 

 solid bodies without change of external form or previous liquefaction ; as will be 

 hereafter explained, this is frequently the case in nature. The structure so developed 

 is, however, very distinct from the crystallisation after liquefaction, characteristic of 

 the eruptive rocks. 



