The operations of denudation and deposition are so completely bound up with the appearances presented by these rocks that it is quite hopeless to attempt their study without a good knowledge of physical geology or petrology. That knowledge will be assumed, and only the lines on which the examination of minerals in thin sections should proceed will be indicated. It is however important to examine hand specimens if they are available. The following are some of the important sedimentary rocks determined in the field of optical mineralogy.

Arenaceous Rocks - are usually pebbly and sandy in hand samples. It has been found in optical mineralogy studies that thin sections of pebbly rocks do not often convey much information, since a section cut from one fragment may differ completely from another on account of the marked heterogeneity of such a rock.

However, sandy rocks may often be examined with profit. As a result of their mode of formation there has usually been a selection of mineral fragments that the bulk of these derived constituents are made up of minerals which are not decomposed by trituration in atmospheric waters. Quartz is by far the most common of these and it is the most important constituent. Other stable minerals may occur but they are much less common. Minerals such as muscovite, tourmaline, garnets, cassiterite, and zircon are the most important. However, in those cases where the accumulation of the detritus to form the deposits was not accompanied by much water action, less stable minerals had a chance of preserving their identity and feldspars make up a considerable proportion of such rocks.

In general, the fragments are more or less angular when evaluated even more closely with the aid of petrographic polarizing microscope, but, in some few cases it is found that they are actually completely rounded. This is to be regarded as an indication of wind erosion and accumulation under desert conditions. The material holding these grains together is commonly silica, calcium carbonate, or iron oxide, and forms a very characteristic feature of such rocks. In a microscopic investigation of such a rock using polarizing microscope, the derived fragments and the cementing material can be determined. Some attempt should then be made to visualize the conditions under which accumulation took place.

            If hand specimen is available, further information as to the nature of the constituents may be obtained if the fragments are examined in the series of liquids for determining refractive indices using polarized microscpes. Such method of examination is especially valuable with regards t the constituents of these rocks since, as all the fragments have been eroded, thee test of form is no longer available.

Argillaceous Rocks – Shales and slates are usually so fine-grained that even the microscope usually fails to reveal much as to their mineral composition. However, some examples seem to show that their constituents are very much like those of arenaceous rocks, the fragments merely being ground down to much smaller dimensions. Crystals of pyrites, of fairly large size, are sometimes found, but these have been formed in consequence of changes which have taken place since deposition.

Calcareous Rocks – These rocks are organic in origin. Although some of them, such as stalactite and travertine, are formed simply by deposition from solution. Oolitic limestones are probably also inorganic in origin, the calcium carbonate of the oolite having been deposited in successive layers around a nucleus. Calcite makes the bulk of such rocks, and may easily be recognized as such by the twinkling effect observed when the section is examined under the petrographic polarizing microscope with the lower nicol only, and by the absence of the crystal form. The organisms of which the rocks are largely built are usually clearly visible when viewed under polarized microscopes and may actually be identified by those who have sufficient knowledge of paleontology. Foraminifera, such as Globigerina and Nummulites, are sometimes important limestone builders. Corals, crinoids, and various mollusks are also of primary importance.

Some of the calcareous rocks consist entirely, or to a great extent, of the mineral dolomite. Although Dolomite resembles calcite in many respects, it may be distinguished by its tendency to occur in well-defined rhombs, and its frequent color zoning that can be noticed more clearly when viewed with the aid of polarized microscope used in optical mineralogy. It has been observed that fossils are not seen in dolomite. Somewhat similar rocks are those consisting partly of calcite and partly of dolomite. Some of these rocks owe their heterogeneity to the fact that they are partially dolomitised limestones, the magnesium having been added since the rock commenced its existence as a limestone.

Siliceous Rocks – There are two methods of formation determined in siliceous rocks. First, the material accumulated as a deposit of the remains of organism with siliceous skeletons, sometimes sufficiently well-preserved to be identified. Secondly, the rock represents a silicified limestone, silica from solution having replaced a pre-existing limestone. In this latter case, the structures of the limestone may be preserved. Tripoli and some cherts owe their formation to the accumulation of siliceous organisms, flint, some cherts, and such rocks as siliceous oolites, to a process of replacement. The silica of these rocks is usually in the form of quartz, but may be chalcedony or opal. It is in the form of opal that woody tissues are so often preserved.

Carbonaceous Rocks – These rocks do not repay examination in thin section. But few of the Carboniferous coals exhibit the remains of the spores and spore-cases of the Lycopods of that period. Other plant tissues are also occasionally revealed by this method of investigation.

The distinction between sedimentary and igneous rocks, as seen under the polarizing microscope used in optical mineralogy, is sometimes a matter of some difficulty. The two classes overlap in rocks known as tuffs. Tuffs are volcanic in origin, but they partake of the nature of sediments, even to the extent of including fossils. A rock with fossils may, if we put into exception the case of tuffs, safely be considered to be of sedimentary origin. Some difficulty is often encountered in the case of the arenaceous rocks, which may show marked resemblance to igneous rocks. This applies particularly to the case of felspathic sandstones, where a cementing material is not always conspicuous. Even bedding is not to be relied upon as a proof of sedimentary origin. This is so because some rhyolites exhibit structures which stimulate that character very closely.