upon the crystalline rocks, particularly along the eastern margin of the plateau country. They will be severally considered in the discussion of the Coastal Plain formations.

THE WESTERN DIVISION.

THE ALGONKIAN PERIOD.

The supposed Algonkian rocks of the western division of the Piedmont Plateau are infolded with the Paleozoic deposits of Montgomery, Frederick and Carroll counties. They consist, so far as has yet been observed, of but a single type, closely resembling the metamorphosed basic volcanic rocks of the Blue Ridge district.

THE BASIC VOLCANICS.-The basic volcanic rocks represented in Montgomery, Frederick and Carroll counties consist of long lenses or bands infolded with the phyllites and crystalline limestones. They follow the general structural lines in a N. E.-S. W. direction. The lenses are more numerous in Frederick county than in the area to the east. The rocks have been highly metamorphosed and show all the essential characters of the "Catoctin schist " of the Blue Ridge district and will be more fully described under that head.

THE CAMBRIAN AND SILURIAN PERIODS.

The western division of the Piedmont Plateau, comprising the larger part of the western slope of Parr's Ridge as far as the Monocacy river, has been described as composed mainly of semi-crystalline rocks of sedimentary origin. These rocks are almost unaltered along their western margin, and present the same characters as the sandstones, slates and limestones of the Blue Ridge and Frederick valley, where their age has been determined by fossils. As they approach the axis of the" fan," however, which has been shown above to be one of the principal features in the structure of the Piedmont Plateau, these schists become more crystalline. Here they stand nearly vertical, and show that the dynamic action has been at a maximum by the greatly contorted condition of the schists and the abundant development of new minerals within them. The slates have become roofing-slates, or

chlorite and hydromica (sericite) schists, often full of ottrelite, rutile, biotite and other new constituents. The limestones have become compact, hard, fine-grained marbles. The geological position of these rocks has only in part been positively proved by fossils, and they are therefore designated on the geological map by different colors.

THE QUARTZITE.-Isolated areas of quartzitic sandstone of supposed Cambrian age are found developed along the eastern side of the Monocacy valley in Frederick county. A single outlier, already referred to, is found in the eastern division in northern Harford county, which is supposed to be of identical age, but this fact has not been as yet fully established. The most extensive deposit of this material occurs in Sugar Loaf Mountain, near the boundary of Montgomery county. Here the sandstone is very homogeneous, fine-grained and compact, and is very light, frequently white, in color. The massive sandstone strata of Sugar Loaf Mountain form a series of antichines overturned toward the west. The formation continues toward the north in a few insignificant sandstone patches, while toward the south it soon disappears beneath the phyllite series. The Sugar Loaf sandstones pass on their eastern side upward by a gradual transition into the overlying deposits, which in their unaltered portion are somewhat shaley.

THE PHYLLITE AND CRYSTALLINE LIMESTONE.-The shaley layers just described pass over into sandy slates, and these again into the succession of sericite and chlorite schists, which compose the mass of the "semi-crystalline " area. Considerable deposits of limestone, which are now highly crystalline fine-grained marbles, also occur, particularly throughout the northern portion of the district in the area to the west and north of Westminster. As these rocks are followed across their strike toward the east they are seen to become more and more contorted, cleaved and faulted. Closely folded and puckered layers are frequent, and the secondary cleavage approaches nearer and nearer to the vertical. The succession of beds is well displayed along the main stem of the Baltimore and Ohio Railway between Araby and Hood's Mills. The alteration, or re-crystallization, of these rocks, attendant upon the increasing disturbance to which they have been

subjected, becomes so great that it is not always easy to distinguish the line of contact between them and the underlying and more ancient crystallines of the eastern Piedmont region. The recent discovery of fossils in the phyllites east of Araby by Mr. Keith proves some of these rocks to be of Cambrian age.

THE TRIASSIC PERIOD.

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 which will be further described under the head of the Newark Formation and the Diabase.

THE NEWARK FORMATION.-The deposits of the Newark formation unconformably overlie the limestone and phyllite which have been above described and cover a considerable area along the western border of the Piedmont Plateau. Beginning as a belt some ten 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 one 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 the Newark formation consist largely of red and gray sandstones and conglomerates of both silicious and calcareous varieties. The finer grained and deeper colored deposits generally have their individual elements united by a ferruginous cement, while the calcareous conglomerate, which is largely made up of rounded limestone pebbles, is generally imbedded in a reddish calcareous matrix. All of the deposits present structures which indicate that they were formed in shallow water; the coarse conglomerates, the ripple-marked surfaces, and the tracks of animals all point indisputably to this conclusion.

THE DIABASE.-The sandstones and shales of the Newark formation, as well as the rocks of earlier age, are found penetrated by dikes of an igneous rock known as 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. It seems probable that the dikes before referred to as occurring in the eastern division of the Piedmont Plateau are of similar origin. The diabase is holocrystalline and is composed chiefly of plagioclase and pyroxene with olivene and magnetite. The rocks penetrated have been at times considerably metamorphosed by the molten rock, which was forced into their fissures, generally with a hardening of the beds by partial solidification and re-crystallization. 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, forming lofty ranges of mountains. 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 acted from the east toward the west, hence 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 of 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 oldest of these sediments are toward the east and the youngest 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 three distinct physiographic and geologic districts, but as the features of each division appear to some extent repeated in that which is adjacent to it, it seems more desirable to treat the geology of the Appalachian Region as a unit, and describe under each formation its distribution, character and structure. Reference to the map will show the relations which these formations bear to the several geographic divisions.

The following divisions are recognized in the rocks of the Appalachian Region.