MELLO: MICROSCOPICAL STRrCTURE OF ROCKS. 
171 
particular geological period present well-marked niiiieralogical or 
microscopical peculiarities which distinguish them fi'om rocks belong- 
ing to the same general group which were simultaneously erupted 
in some other pro^-inces." 
Having thus briefly noted the characters of some of the principal 
features of the igneous rocks and their associated minerals, as 
exhibited by the microscope, we may pass on to a short consideration 
of the general microscopical characteristics of the aqueous rocks. 
In looking at these we shall be at once struck with the very 
different appearance presented as compared with those of igneous 
origin. Take for instance a thin section of a sandstone, no matter 
how fine-grained it may be, we shall at a glance perceive it to be an 
aggregation of particles, more or less rounded and water-worn, of 
quartz and other minerals, evidently derived from the breaking up of 
other rocks. Clays, shales, and most of the slates present a some- 
what similar appearance, although with a more minute structure, 
which in the slaty rocks is sometimes considerably modified by the 
effects of pressure, as well as by other agencies, since their 
first deposition as sediments. Again the microscope will in many of 
these aqueous rocks bring to view traces more or less distinct of 
organisms. Some of the clays and shales, and especially the lime- 
stone, will be found to contain foraminifera, diatomaceie, and other 
minute forms of the life of the geological period to which they belong. 
The aqueous rocks may be conveniently divided into two groups, 
the non-calcareous and the calcareous. 
The former have been derived from the breaking up of rocks of 
earlier date, in the first instance those of igneous and metamorpliic 
origin, thus when we come to examine the materials of which they 
are composed, we find that they are the same minerals for the most 
part with which we have ah-eady been made familiar in our study of 
the plutonic and volcanic rocks, only connninuted, broken, and more 
or less rolled and water-worn, and sometimes corroded and partially 
decomposed. A careful study of the individual particles of a 
sedimentary rock will enable us very often to trace its origin. 
The common non-calcareous rocks are the sandstones and giits, and 
* Judd, Q. J. G. S.. vol. xlii., pi. 1. 
