minute crystallites are more common. Chlorite sometimes occurs as a decomposition product of the hornblende.

GRANITE

Granite occurs in long, narrow belts with a northeast-southwest trend in areas north and south of the main gabbro belt. (Plate I.) Four isolated areas are found along the Susquehanna near Lapidum and Havre de Grace.

As a rule the granite is a light-gray to white rock, although in places segregations of biotite may give it a much darker color. The color is decidedly lighter than that of the average Baltimore gneiss. At times the granite develops a definite gneissic structure. This structure, however, is probably due to the formation of secondary muscovite in plates perpendicular to the direction of application of the dynamic forces. Nowhere is this gneissic structure as pronounced as in the Baltimore gneiss. At other times the granite presents a very massive appearance, no gneissic structure being visible. Megascopically, feldspar, quartz, biotite and muscovite are easily distinguished. Under the microscope the feldspar is found to be principally an acid plagioclase, which extinction angles show to be of the albite-oligoclase series. A subordinate amount of orthoclase is also present. The mica is usually considerably smaller in quantity than the feldspar. Biotite occurs in irregular plates. Most of the muscovite is probably secondary, although a small amount is undoubtedly primary. Hornblende is sometimes found in small quantities, but never sufficient to be considered anything more than an accessory constituent.

DIABASE

Although a number of diabase dikes of considerable extent have been found intruding both the gabbro and the granite in Cecil County, thus far only four short diabase dikes have been mapped in Harford County, all of them within the main gabbro belt. These dikes differ from those in Cecil County in the amount of metamorphism to which they have been subjected. Where outcrops are not available the diabase dike usually

reveals its presence by the number of cubical boulders scattered about the surface. These boulders almost invariably possess a coating of rusty yellowish-red material caused by weathering. Where exposures show the contact between gabbro and diabase it is found to be clear and distinct. There is no transition zone between the two types of rock, and no contact metamorphism. When a fresh specimen of diabase can be obtained it shows a greenish-black color with very fine grain. Usually the individual mineral grains cannot be distinguished without the aid of the microscope. Under the microscope it is at once apparent that the rock has been subjected to considerable metamorphism, although the ophitic texture is still plainly visible. The occurrence of the plagioclase indicates that it was the first mineral to crystallize. Plagioclase crystals have lath-like forms with idiomorphic shapes. The other chief constituent of the rock is a green, usually compact hornblende which acts as a matrix for the plagioclase. In a few cases remnants of pyroxene are found enclosed in the hornblende. The relation of the pyroxene to the hornblende shows that the latter mineral is a secondary alteration product of the former. The hornblende often contains minute crystallites of a clear, colorless mineral arranged parallel to the cleavage lines. Because of their small size it is impossible to determine accurately the optical properties of these bodies, although they correspond quite closely to those of quartz. In the majority of cases the plagioclase crystals have been altered to an aggregate of epidote and zoisite. These aggregates are so arranged that the ophitic texture has still been preseverd. Determination of the extinction angles of the unaltered plagioclase gives values corresponding to a composition of about Ab25An75. The plagioclase thus belongs to the labradorite-bytownite series. Magnetite is frequently present in small isolated grains. The presence of a small amount of chlorite in a few of the sections indicates some weathering of the hornblende.

The diabase dikes of Cecil County are very similar in occurrence to those of Harford County. In Cecil County, however, these dikes have been subjected to much less metamorphism than those of Harford

County. Leonard29 states that the diabase of Cecil County is composed of lath-shaped crystals of feldspar, irregular grains of augite, and a little magnetite; and Bascom30 says that the constituents are "remarkably fresh," although she notes some alteration of pyroxene to green hornblend and chlorite. Instances of the alteration of the usual constituents of diabase to hornblende, chlorite and epidote are very common in the literature. Williams31 has carefully described diabasic rocks from the Menominee and Marquette districts of Michigan with welldeveloped ophitic texture in which the augite has been altered to fibrous or compact green hornblende and the feldspar to epidote or saussurite. Finckh32 has noted a diabase with ophitic texture intrusive into the gabbro of northern Syria, in which pyroxene has been uralitized and feldspar altered to epidote. Similar alterations of ophitic diabase have been described by Loughlin,33 Palache, and many others.

34

CONTACT PHASES

Along or near the southern border of the main gabbro belt there are several small districts in which rock types are found with compositions and textures quite different from the principal rock types of the region. The rocks of these districts will be considered in detail.

One of these districts lies about half a mile to the east of Berkley near the west bank of the Susquehanna. It has an east-west trend, and is not more than an eighth of a mile in width nor more than a half-mile in length. The predominant rock of this area is a quartz epi-diorite, that is, a rock having the mineralogical composition of a quartz diorite, but in which some of the essential constituents are secondary alteration products. The transition between this rock and the hypersthene gabbro to the north is concealed under the cover of soil. Outcrops are few and far between. Megascopically the rock is holocrystalline,

29 Leonard, A. G., Sm. Geol., vol. xxviii, 1901, p. 155.

30 Bascom, F., Mary and Geol. Survey Cecil County, 1886, p. 141.

31 Williams, G. H., U. S. Geol. Survey Bull. 62, 1890, pp. 138-146, 168-175.

32 Finckh, L., Zeits. deutsch. geol. Gesell., Bd. L, 1898, p. 143.

33 Loughlin, G. F., U. S. Geol. Survey Bull. 492, 1912, p. 102.

14 Palache, C., Univ. Cal. Dept. Geol. Bull., vol. I, 1896, p. 174.

medium to fine-grained, with a greenish-gray color. In the hand specimen clear, glassy, bluish quartzes, feldspar (both clear and milky), hornblende, biotite, and a small amount of pyrite can be seen. Under the microscope the rock is seen to be composed chiefly of quartz, basic plagioclase, saussurite, chlorite, hornblende and pyrite. The feldspar is in part basic plagioclase, usually showing zonal development. Measurement of extinction angles shows the interior to have a composition of labradorite-bytownite, while the outer zone has a composition of andesine-labradorite. Usually the feldspar has been more or less completely altered to a mixture of epidote and secondary untwinned albite. The large quartz grains are either single individuals or they have been crushed into mosaic aggregates. Undulatory extinction is very common, both in the large grains and in the particles making up the mosaics. Green hornblende is another of the principal constituents, sometimes making up half of the rock. Although no pyroxene can be detected in the section the hornblende has all the characteristics of a secondary mineral. The fibrous variety is very prominent. Chlorite also occurs in considerable quantities, evidently produced as a decomposition product of the hornblende. Biotite is present only in isolated particles.

South of this district meta-gabbro, consisting mostly of amphibole and chlorite, makes its appearance. This is succeeded by the ordinary meta-gabbro, and just to the south of this is found a rock which apparently marks a transition zone between ordinary hypersthene gabbro and quartz epi-diorite like that half a mile to the north. Megascopically this is a greenish-gray rock of varying grain in which can be seen glassy quartz grains (as large as inch in diameter), greenish-black amphibole, and milky feldspar. The microscope shows the rock to be composed of saussurite, augite or diallage, quartz, chlorite, hornblende, phlogopite, zircon, and garnet. The saussurite is made up of large secondary albite crystals, usually without twinning, which act as a matrix for great numbers of crystallites of epidote and zoisite. Diallage is present in various stages of alteration to hornblende and chlorite. Chlorite, particularly, is very abundant in this rock. Quartz is either

in large grains, often crushed, or in veinlets with mosaic fabric. Phlogopite is fairly abundant. It is recognized by its occurrence in irregular plates with low index of refraction, fairly high double refraction, very small optic angle (practically 0°), and negative optical character. Small crystals of zircon, characterized by good crystal form, very high index of refraction, high double refraction, and positive uniaxial character, are frequently found. Garnet is also a common accessory constituent of the rock. The presence of phlogopite and garnet indicates the influence of a kind of contact action between the typical hypersthene gabbro and this more acid phase.

Because of soil covering there is no continuous outcrop, and therefore the gradations between the transition rock and the quartz epi-diorite itself cannot be observed. About a quarter of a mile south of the transition phase there is an exposure showing material very similar in character to that of the quartz epi-diorite in the area east of Berkley. Megascopically this rock is grayish-green with phenocrysts of clear, glassy quartz grains as large as a quarter of an inch in diameter. The groundmass is green with a decidedly laminated appearance. Under the microscope the rock is seen to be composed of quartz, saussurite, chlorite, hornblende, biotite, pyroxene, and pyrite. Quartz usually occurs as a mosaic aggregate, although single large grains are sometimes seen. Saussurite here consists of epidote and secondary albite, the albite acting as matrix for the epidote. Chlorite is present in fairly large quantities, evidently the result of the weathering of secondary hornblende. Green hornblende, usually fibrous, is quite common in this rock. The thin section sometimes shows a remnant of diallage surrounded by this fibrous hornblende, although in most instances the pyroxene has entirely disappeared. The area itself is wedge-shaped and half a mile wide at its broadest part. The contact with the Baltimore gneiss to the south is not exposed, but it seems very probable that this phase continues to the contact with the Baltimore gneiss without a recurrence of the typical hypersthene gabbro.

Another type of rock, evidently the result of contact metamorphism, is found near the border of the gabbro area just south of Darlington.