03 THE MICROSCOPICAL STRUCTURE OF ROCKS. 
5 
such a rock to be of purely igneous origin. All the old trap- 
pean rocks, the more recent basalts and pitchstones, modern 
trachytes and lavas, are all of them found to agree in character ; 
and by the nature of the cavities alone, which are revealed by 
the microscope, in their component crystals, they are proved to 
have had a common origin from a state of igneous fusion. 
Occasionally we find in some of these rocks large cavities, 
that have been filled up with zeolites and with calcite, and in 
these are found water cavities, but no stone or gas cavities ; and 
the conclusion thence arrived at is, that the rock has been ex- 
posed to water since its first formation, and its larger cavities 
have been filled up by deposits held in solution by that water. 
Mr. S or by has called attention to certain blocks ejected from 
Vesuvius during eruption, which differ from the lavas in con- 
taining many minerals not found in these latter. Amongst 
these blocks are some of limestone, which has been torn from 
the strata through which the volcano has burst, and in the 
minerals enclosed in these blocks are found many fluid and gas 
cavities, as well as stone and glass cavities ; and this, taken in 
combination with certain experiments connected with the 
nature and fusibility of the contents of some of these cavities, 
leads to the supposition that the crystals of the ejected blocks 
were produced at a red heat in the presence of melted stony 
matter, gases, vapours, and liquid water, saturated with various 
salts, so that fusion, sublimation, and solution all had a share in 
their formation. An examination of granitic rocks reveals a 
structure having some features closely analogous to that of these 
blocks from Vesuvius. The quartz of these granites is seen to 
contain cavities of all sorts, fluid, stone, and gas ; and this seems, 
to show very clearly that the rocks of the granitic series of all 
dates originated in a similar manner, that they also were de- 
veloped 64 by the combined influence of a dull red heat, liquid 
water, and partially melted rock,” or, in other words, 64 igneous 
fusion, aqueous solution, and gaseous sublimation.” 
The quartz and other minerals so often found in mineral 
veins would appear to have been deposited in fissures, up which 
the highly heated water containing them in solution was driven 
by the heat from below, and in which on its cooling they would 
be crystallised. 
II. 
We will turn now to another branch of our subject, and 
notice, in the next place, how much help is afforded by the 
microscope in determining the nature of the minerals com- 
posing rock masses ; minerals often present, either in such small 
quantities or in such minute forms that the eye alone is insuffi- 
