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POPULAR SCIENCE REVIEW. 
course, entirely dependent upon the transparency of the rock 
constituents, and is commonly from to of an inch. 
Thin splinters of rocks and powdered fragments, mounted in 
Canada balsam, may also be examined with advantage, but can- 
not replace the above-described sections. 
The examination of such a rock section enables a mineralogi- 
cal analvsis to be made, even of the most compact and apparently 
homogeneous rock, and generally leads to the discovery of other 
mineral constituents previously unsuspected, from their being 
invisible to the eye, and also, as Sorby has observed, allows 
those minerals, formed at the time of solidification of the rock, 
to be distinguished from such as are the products of subsequent 
alteration. 
Arranging rock species according to their structure, it will be 
found that most rocks fall naturally into one or other of two 
great classes — 
I. Primary or Eruptive Rocks ; 
II. Secondary or Sedimentary Rocks; 
and it will be seen that the microscope is of special value when 
applied in cases where the external appearance renders it 
doubtful as to which of these classes a rock may pertain. 
The terms 'primary and secondary are here used quite inde- 
pendently 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. Primary or Eruptive Rocks. 
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 regarded as having been once a molten sphere, the 
consolidated original crust of the globe would pertain to this 
class of rocks. 
Mineralogically they consist of crystallised silicates, with or 
without free quartz, and usually containing many other minerals 
in minor quantities, especially metallic compounds, as magnetite, 
* These rocks are indiscriminately called volcanic, igneous, plutonic, cry- 
stalline, &c. The term crystalline, although characteristic of these rocks, 
is not exclusively so, and is consequently less appropriate ; many normal 
sedimentary beds, as rocksalt, gypsum, &c., are perfectly crystalline, and 
others when altered by metamorphic action, become more or less so. 
