MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
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rock. Whether it is a purely aqueous deposit, or the result of igneous 
fusion, whether it has been crystallized from watery solution or after 
gaseous sublimation, or out of simply molten materials ; or whether 
all these agencies, igneous fluidity, sublimation and heated water 
have at once or successively taken part in its formation. These are 
the questions to which we are beginning to receive answers from the 
use of the microscope, and the value of this instrument in geological 
investigations is daily becoming more and more recognized, so much 
so, indeed, that no description of a rock can be considered complete, 
without some record of its microscopical structure. 
For the purposes of examination, we may consider the rocks as 
divided into two great classes Aqueous and Igneous, there are 
others of more or less doubtful character, rocks which may indeed 
belong to either of the above classes, but which have undergone 
alterations subsequent to their original formation. 
Professor Bonney noting an objection raised to sharp lines of 
demarcation, says that "it is impossible to draw a hard and fast line 
" between igneous and sedimentary rocks, because the former are 
" frequently only the result of metamorphosis of the latter carried to 
an extreme degree, so that the one series ^ •iss' -gradually into the 
" other " at the same time points out that, for all practical purposes, 
this need not be any real difficulty, the majority of rocks can be 
grouped around certain types, and there is also a marked difference 
as a rule between the rocks classed as igneous and the sedimentary, a 
difference not merely in appearance and structure, but also what is 
more important, in chemical composition. By an igneous rock we 
mean one " that has solidified from a state of fusion due to the 
" existence of an elevated temperature, whether we may call this dry 
" fusion or not." 
The great mass of the aqueous rocks are those commonly known 
as the "stratified rocks", having been thrown down as sediments or 
precipitates, or elaborated by organic agency, and which occur in an 
orderly succession of beds, whilst the igneous rocks are more irregular 
in their mode of occurrence, they may underlie the stratified 
rocks, or they may pierce through them, forming dykes or veins, or 
on the other hand, they may be interbedded with some of them and 
