THE MICROSCOPE IN GEOLOGY. 
363 
bewildered under such circumstances, and inclined to settle 
down in the comfortable belief of the transmutation or transition 
of sedimentary rocks into eruptive, &c., and even the chemist 
feels puzzled, when he finds that a rock taken out of apparently 
normal stratified deposits has the same chemical composition 
with one of undoubtedly intrusive nature. The microscopic 
examination, however, soon shows that, however similar the 
external appearance of two such rocks might be, their internal 
structure is totally different ; showing in the primary rock the 
crystallised structure and arrangement previously described, 
whilst the secondary rock is resolved into a mere agglomeration 
of more or less broken fragments of the same minerals consti- 
tuting the former. In beds formed from the consolidation of 
volcanic ashes, the microscopic examination occasionally affords 
evidence as to whether such ashes had been deposited on land, 
or had fallen into water. 
2. Rocks built up of the more or less rounded or angular 
debris of previously existing sedimentary or eruptive rocks . — 
Where sufficiently coarse-grained, these rocks constitute ordinary 
conglomerates, breccias, grits, sandstones, &c., and are easily 
analysed by the eye; but if fine, as shales, slates, &c., the 
microscope must be appealed to, in order to resolve them into their 
constituent mineral or rock particles, and by this means it will 
be seen that even the most compact and homogeneous specimens 
are a mere aggregate of more or less rounded and water-worn 
grains of quartz, weathered felspar, mica, chlorite, soft and hard 
clays, clay slate, oxide of iron, iron pyrites, carbonate of lime, 
fragments of fossil organisms, &c., arranged without any trace 
of decided structure or crystallisation, even when the highest 
powers of the microscope are employed in their examination. 
The physical structure and optical properties of the mineral 
components enable them, however, to be recognised with great 
certainty, even when in grains of less than YoVo an ^ EL 
diameter. 
PL XVIII. fig. 14, shows the appearance of a section of a fine- 
grained (uncleaved) Silurian clay slate from Sorata, in Bolivia, 
magnified 400 linear. This rock is composed of irregular grains 
of quartz sand, weathered felspar, and waterworn mica, along 
with specks of oxide of iron and iron pyrites, all promiscuously 
mixed. 
In the case of roofing slate, however, the microscope shows 
that the constituents, instead of being distributed at random 
throughout the mass, possess a definite arrangement, as may be 
seen in the section of lower Silurian roofing slate from the 
Festiniog quarries (PL XVIII. fig. 15), where they are disposed 
in parallel lines, thus constituting lines of weakness or the cleav- 
age of the slate. The researches of Sharpe and Sorby have con- 
