presence is usually indicated by an abundance of boulders or white chalklike streaks in the roadside cuttings. They are abundantly developed in the valley of the Patapsco in Baltimore, Howard, and Carroll counties and along the Susquehanna in the vicinity of Castleton and Conowingo, where they are of sufficient size and purity to become of some commercial importance as the source of feldspar and "flint" used in pottery works. THE ACID VOLCANICS (META-RHYOLITE).-Along the lower Susquehanna gorge in the vicinity of Frenchtown and Havre de Grace are a few dark colored rocks of greenish hue due to their contained epidote. These "greenstones" occur in small dikes cutting the adjoining rocks and may be traced away from the river on either side but especially to the eastward in Cecil County. They are of little areal importance but are of considerable interest since they represent almost the only evidence of former volcanic activity in the eastern district of Maryland.

THE MESOZOIC DIABASE intrudes all of the older crystalline rocks in a series of dikes that may be traced with occasional interruptions across the entire eastern district of the Piedmont. The rock is a typical Mesozoic diabase presenting no unusual features. It is seldom well-exposed, the courses of the dikes being marked by deep red soil and dark red or brown rounded boulders of " trap."

THE WESTERN DISTRICT

The rocks of the western district, while much less thoroughly known than are those of the eastern district, show among themselves a sequence which is strikingly in harmony with that discovered in the eastern district. On the eastern side of the western district the rocks are scarcely distinguishable from those of the eastern district, but in the western part of the district the rocks are much less metamorphosed. There the limestones of the Frederick Valley and the underlying quartzite of the mountains are indistinguishable from the corresponding rocks on the western side of the Blue Ridge. Moreover, the limestones are fossiliferous and show clearly that their age is the same as that of the limestones of the Shenandoah Valley. The rocks of the eastern side of the district are progressively more metamorphosed eastward and are practically devoid of fossils. The

natural interpretation is that these rocks are younger than the limestones of the Frederick Valley, since the latter are dipping to the eastward beneath the mass of micaceous argyllites and sandstones. The frequent appearance of limestone valleys indicates that the structure is not simple. and the finding of volcanic rocks similar to those of the Blue Ridge requires that judgment regarding the actual age of these eastern rocks be held in abeyance until the area has been mapped in detail.

At the present time it is possible to recognize the following types of rocks:

FORMATIONS OF THE WESTERN PIEDMONT PLATEAU

THE QUARTZITES AND SCHISTS.-Areas of quartzite, chiefly in ridges more or less timbered, have been occasionally found in the preliminary study of the western district of the Piedmont. They appear to be similar lithologically to the Weverton sandstones of the mountains on the west. The stratigraphic position of these siliceous rocks with relation to the marbles and limestones has not been established. At times they appear to be above the latter and at other times below them.

THE MARBLES.—The highly calcareous rocks here referred to embrace those lying east of the Frederick Valley. They are more crystalline than are those of the latter locality and are, so far as is known, devoid of

fossils. Whether or not they are local areas of more highly metamorphosed limestones of Shenandoah age has not been definitely settled, but this conclusion appears to be the one naturally suggested by the areal distribution of the marble exposures.

These marbles, which are frequently beautifully colored, occur in long narrow valleys, especially in the region of the Western Maryland Railroad. Exposures are few, owing to their relative weakness, and most of the areas underlain by these rocks are excellent farming lands.

THE SCHISTS AND ARGYLLITES.-Clearly overlying the marbles and separated from them by unconformities are certain schists and argyllites, part of the mass formerly called phyllites.

THE WEVERTON SANDSTONE.-Isolated areas of quartzitic sandstone similar to the sandstone of the mountains are developed along the eastern side of the Monocacy Valley in Frederick County. The most extensive deposit of this formation occurs in Sugar Loaf Mountain, near the boundary of Montgomery County. Here the sandstone is very homogeneous, fine grained, and compact, and very light in color.

THE SHENANDOAH LIMESTONE.-Along the extreme western border of the plateau country in the Frederick Valley is an extensive development of Shenandoah limestone similar to the more extensive development of this formation in the Hagerstown Valley.

THE NEWARK FORMATION.-The rocks of Triassic age are mainly confined to the western margin of the Piedmont Plateau and are represented by both sedimentary and eruptive materials. The deposits of the Newark formation unconformably overlie the limestone and phyllite which have been above described and cover a considerable area. Beginning as a belt some 10 miles in width in northern Carroll and Frederick counties, the formation gradually narrows toward the south, until in the region of Frederick its full width does not exceed 1 mile, while at one point directly to the west of Frederick the continuity of the beds is completely broken. Farther southward in western Montgomery County the belt of Newark deposits again broadens to a width of several miles.

The rocks of this formation consist largely of red and gray sandstones and conglomerates of both siliceous and calcareous varieties.

Igneous Rock's

THE BASIC VOLCANICS in the Piedmont and in the mountains seem to be remarkably alike. They are essentially bluish-green with white masses of epidote and quartz which on exposure become dull gray or yellow. The honeycombed or amygdaloidal character of many of the masses increases the individuality of these rocks which usually form rough surfaced ledges or areas thickly strewn with characteristic boulders. The original character of these basic volcanics was that of a diabase or andesite, but the subsequent metamorphism which they have suffered has often rendered them schistose and obscured their original appearance.

THE ACID VOLCANICS.-When only slightly metamorphosed, as in certain localities in the Blue Ridge, the acid volcanics are close-grained, dark bluish-gray or purplish rocks, frequently speckled with small feldspar phenocrysts. In the Piedmont occurrences, however, these highly siliceous volcanics have usually been modified by pressure until, in extreme instances, they are fissile slates. In the quarry, they may appear solid and massive and flecked with feldspars, but even here the schistosity is generally evident.

These same rocks occupy considerable areas in the mountains, but here their higher altitude and inaccessibility render them less important. The unconformity between these phyllitic meta-rhyolites and the overlying rocks is seen by the frequent absence of one or more of the younger formations.

THE DIABASE.-The sandstones and shales of the Newark formation, as well as the rocks of earlier age, are found penetrated by dikes of the igneous rock diabase. These dikes extend across the area, for the most part, in a north-south direction, and throughout central Frederick and Carroll counties, where the covering of sandstones and shales has been removed, are found penetrating the limestones and phyllites. The diabase decomposes with considerable rapidity, although the surface is generally covered with large boulders of undecayed material which show characteristic weathering.

THE APPALACHIAN REGION

The geology of the Appalachian Region, as in the case of the Piedmont Plateau, cannot be fully comprehended without taking into consideration the great belt of which it forms a part. The beds of sediments which form the limestones, sandstones, and shales of the Appalachian mountains were deposited in a wide, long trough, which once extended from north to south throughout the region now occupied by the mountains. This trough was undergoing gradual depression through most of Paleozoic time, until many thousands of feet of conformable beds had accumulated in it, mainly as the debris of a continental mass lying to the east.

This vast accumulation, at the close of Paleozoic time, was so compressed as to be forced up into a series of great folds. The present Appalachians are merely the remains of these ancient folds worn down by natural processes through many successive periods. It is by no means certain that the mountain crests ever stood higher than at present, for from the moment the land rose above the sea the forces of denudation became active, and with varying intensity have continued to the present day. The great folds have been from time to time planed down, to be again sculptured as the result of elevatory movements. The compressive force which raised these mountains probably acted from the east toward the west, since the most intense disturbance is always observable in the eastern portion of the range and dies away gradually into the central plains. A secondary result attributed to this action from the east, is that all the folds are tipped toward the west and all the great faults show a thrust in the same direction. In consequence of this the older sediments are toward the east and the younger toward the west, although the more or less abrupt folds into which they were thrown, when raised into a mountain chain, have since been cut off by erosion in such a manner as to show a repeated succession of strata and at the same time to present in portions of the eastern border area rocks of still earlier age. The section made by Maryland across the Appalachian system between the Frederick Valley and the western line of Garrett County presents an almost complete series of these various formations. As has been already pointed out, the mountain system of Maryland is divisible into