=34. Aeolian Rocks.=—This class of rocks is of less importance than the others, either geologically or agriculturally. It is formed from materials drifted by the winds and this material has various degrees of compactness. Usually the components of these drifts form rocks or deposits of a friable texture and of a fragmental nature. The very extensive deposits of loess in China, forming their most fertile lands, are admitted now to have been formed in this way, but it is now generally admitted that similar deposits in this country are of subaqueous origin. Chief among these rocks, are the volcanic ashes which are often carried to a long distance by the wind before they are deposited and consolidated into rock masses. Many loose soils may be carried to great distances by the wind, the deposits forming new aggregates. This is particularly the case in arid regions. =35. Metamorphic Rocks.=—This class of rocks includes all sedimentary or eruptive rocks, which, after their deposition and agglomeration, have been changed in their nature through the action of heat, pressure, or by chemical means. Sometimes these changes are so complete that no indication of the character of the original rocks remains. At other times the changes may be found in all the stages of progress, so that they can be traced from the original fragmental or irruptive to the completely metamorphosed deposit. This is especially true of rocks containing large quantities of lime. In those containing silica, the changes are less readily traced. [Illustration: FIG. 8. MICROSTRUCTURE OF CRYSTALLINE LIMESTONE. (West Rutland, Vermont.) ] The metamorphic rocks may be divided into two subclasses, namely, stratified or bedded, and foliated or schistose. The rocks of the first class are represented by the crystalline limestones and dolomites. The microstructure of a crystalline limestone is shown in Fig. 8.[25] When lime and magnesia occur together in combination with carbon dioxid, the substance is known as dolomite. The chemical nature of these rocks and their soil-forming properties are the same as those of the ordinary, non-metamorphosed limestones and dolomites to which reference has already been made. The subject need not, therefore, be further dwelt upon here. [Illustration: FIG. 9. MICROSTRUCTURE OF GNEISS. (West Andover, Massachusetts.) At _a a_ are shown plagioclase crystals broken and rounded by the sheering force producing the foliation. ]