The augen-gneisses, so often found near the epi-diorite bodies, show every evidence of having been derived from the Baltimore gneiss. The feldspar is an acidic plagioclase, quite characteristic of the gneiss. No basis plagioclase is present. Quartz, which is always abundant in the augen-gneiss is also characteristic of the Baltimore gneiss, while it is very rare in the gabbro. The exceedingly well-developed gneissic structure at once indicates a close relationship to the Baltimore gneiss. In places the formation of eyes is only suggested, although the feldspathic materials are concentrated into lenses. The segregation of feldspathic bands is not uncommon in the Baltimore gneiss. Tourmaline is no doubt the result of pneumatolytic action. The development of chlorite in the augen-gneiss is very similar to a development of a chlorite schist noted by Chatard at the contact of a dunite with hornblende gneiss. It is impossible to say what the original mineral was, from which the chlorite has been formed. Since hornblende is often a very a bundant mineral in the gneiss, it seems quite probable that chlorite is a decomposition product of hornblende in this instance. The evidence of the structure and mineral composition of the augen-gneiss indicates that the heat of the intrusive body had raised the temperature and increased the activity of solutions within the gneiss. These solutions have deposited material around centers already present and thus the feldspar eyes have been built up. Augen-gneisses built up by such a process of deposition from solution have often been described. Sometimes this has been accomplished in schists or gneisses without the aid of heat from a neighboring igneous body. Berg37 describes an augengneiss formed by the production of new quartz crystals between schist lamellae by deposition from circulating waters. Fenner38 mentions an augen-gneiss in the Highlands of New Jersey, concerning which he says: "Certain phenomena suggest that a force of crystallization may at times have been an effective factor in separating the layers. Aggregates of microcline and quartz crystals in the form of augen are seen,

36 Chatard, T. M., U. S. Geol. Survey Bull. 42, 1888.

37 Berg, G. E. W., K. Preuss. geol. Landesanst. Jahrb., Bd. xxviii, 1907, p. 647. 38 Fenner, C. N., Jour. Geol., vol. xxii, 1914, p. 605.

inclosed by curving bands of schist." Fenner concludes that the preCambrian gneiss of that region was caused by the injection of a thinly fluid granitic magma between the layers of an original rock of laminated structure. Williams has described a case of contact metamorphism in the rocks of the Cortlandt series near Peekskill, New York, where mica schists have been greatly affected by the intrusion of a mica diorite. At some distance from the contact the mica schists are highly crystalline, but not much crumpled or metamorphosed. "As we follow them, however, across the strike they become more and more puckered and filled with lenses or eyes of quartz containing garnet and other minerals. In the schists themselves are developed staurolite, cyanite and garnet. The intensity of the metamorphic changes is directly proportional to the nearness of the schists to the massive rocks." Judd has noted many instances of increase in the growth of feldsgar in massive rocks after solidification. He says (op. cit. p. 185):-"That, as a result of contact metamorphism, many well-defined mineral species are developed in the midst of solid rocks, the crystals growing at the expanse of and deriving their materials from the surrounding detrital fragments, has long been recognized." Harker in a study on the migration of material during the metamorphism of rock masses concludes that the size of individual crystals of secondary minerals increases toward the contact with the intrusive rock. This, he thinks, is due to the fact that migration of material within a rock mass undergoing metamorphism has more latitude when the temperature is higher. Van Hise12 thinks that contact effects are often brought about by heat and increased action of circulating waters near a molten magma. Not only the formation of the feldspar eyes but also the pronounced development of chlorite in the Harford County augen-gneiss was probably caused by the increased activity of the heated solutions.

The augen-gneiss one mile south of Gibson seems by its location to be more of an inclusion in the gabbro than a true contact rock. The

"Williams, G. H., Am. Jour. Sci. (III), vol. xxxvii, 1888, p. 255.

40 Judd, J. W., Quart. Jour. Geol. Soc. London, vol. xlv, 1889, p. 182.

1 Harker, A., Jour. Geol., vol. I, 1893, p. 577.

42 Van Hise, C. R., Mon. U. S. Geol. Survey, vol. xlvii, 1904, p. 488.

lack of exposures between this place and the Baltimore gneiss, however, makes the location of any definite contact impossible.

From a review of the evidence given above, therefore, it is very probable that the augen-gneiss is a phase of the Baltimore gneiss produced by the heating and increased activity of solutions within the Baltimore gneiss because of the proximity of the molten gabbro magma. together with the action of some pneumatolytic solutions from the gabbro magma.

Contacts of the granite with the gabbro are not usually visible, but good exposures of gabbro and granite within a very few feet of each other have been observed. Nowhere has any decided contact metamorphism of either the gabbro or granite taken place. In one place granite has intruded the gabbro in the form of a relatively narrow dike about a mile long. Some of the specimens from this dike appear slightly harder and less friable than specimens of granite collected from other exposures in the region. There has been no change because of contact action in size of grain, texture, or kinds of minerals developed in the granite.

On a geologic map of Harford County published by the Maryland Geological Survey in 1904, the gabbro is shown extending across the county line into Baltimore County. Examination of exposures at or near the county line shows that the gabbro belt dies out or is covered by Baltimore gneiss before it reaches the county line. The Baltimore gneiss in this region is a very hornblendic, dark-colored phase, sometimes quite massive in appearance and often resembling gabbro very closely. A description of these rocks has already been given. The composition of the feldspar, the arrangement of the hornblende crystals, the abundance of quartz, and the structure indicate that the rocks here are to be classified as Baltimore gneiss and not as hypersthene or meta-gabbro.

RELATION TO BASIC INTRUSIVES OF ADJOINING AREAS

While in many respects the hypersthene gabbro of Harford County is very similar to the gabbros described from Baltimore County and Cecil

County, Maryland, Delaware and Pennsylvania, it is lacking in some phases characteristic of these other areas and shows some phases that thus far have not been described from the other areas.

Williams was the first to describe gabbros from the region around Baltimore. Gabbro occurs in the Baltimore region in a few areas of irregular shape. Hypersthene gabbro is found very similar to the hypersthene gabbro of Harford County, in which bytownite, hypersthene and diallage are the only essential primary constituents. Intimately associated with hypersthene gabbro is gabbro-diorite, which has essentially the same physical characteristics and the same mineral content as the meta-gabbro of Harford County. The gabbro-diorite has been produced from the hypersthene gabbro through the alteration of pyroxene to hornblende by the action of regional metamorphism. The gabbro-diorite often shows the alteration of feldspar to saussurite as well. Peridotites and pyroxenites are associated with the gabbro, but are a younger eruption. Gabbros and pyroxenites and peridotites were probably derived from the same parent magma. Williams has described one specimen of olivine gabbro from the Baltimore region. So far no olivine gabbro has been found in Harford County.

The basic intrusives of Cecil County have been described by Leonard and by Bascom. The main gabbro area in this county is a continuation of the gabbro belt of Harford County. There are also a few smaller isolated areas of gabbroic rock. Here are found hypersthene gabbro and meta-gabbro (gabbro-diorite) so prominent in Baltimore and Harford counties, although norite is the predominant phase in Cecil County. The norite is a hypersthene-bytownite rock with no essential diallage. Norite and hypersthene gabbro belong to the same intrusive mass, and are simply differentiation phases of that mass. Two specimens of olivine norite have been described by Leonard. Intrusions of pyroxenite and peridotite are found. Here too they are seen to be younger than the gabbro. Dikes of diabase, pyroxenite, peridotite and pegmatite cut the norite and gabbro. Diorites, very similar in composition to the epi-diorites of Harford County, form. a narrow strip along the southern border of the gabbro. These rocks have been described.

The belt of gabbro continues from Cecil County, Maryland, through Delaware. The gabbros of Delaware have been described by Chester, who found in them several phases quite different from the gabbros of Maryland. In all of the gabbros and associated rocks of Delaware. quartz is an abundant accessory constituent. The predominant type is hypersthene gabbro quite similar to that of the Maryland areas, with the exception of the presence of quartz. By addition of brown hornblende which Chester thinks may be original the hypersthene gabbro passes into a gabbro-diorite. This gabbro-diorite becomes hornblende gneiss with the imposition of a gneissic structure. By the addition of biotite and quartz as important constituents the hypersthene gabbro becomes "gabbro-granite." This may be closely related in composition and origin to the quartz-mica epi-diorite of the Harford County region. The "norite" which Chester has described is a rock "composed mainly of a very fine-grained mixture of quartz and plagioclase, which, in hand specimens, appears quite homogeneous; in this ground mass are porphyritically developed larger grains of hypersthene and magnetite with generally a few shreds of biotite" (p. 22). The gabbro-diorites (metagabbros) of Iron Hill, Delaware, as described by Chester, are very similar to those described by Williams from the Baltimore region. The alteration of feldspar to epidote in the Delaware region has also been noted.

Gabbros closely related to those of Maryland have been described by Bascom from the region included in the Philadelphia folio. The area mapped as gabbro includes norite, hypersthene gabbro, and true gabbro. These rocks are differentiated by the presence or absence of hypersthene or diallage. In all three forms of gabbro, labradorite or labradoritebytownite is the usual feldspar present. This region is exceptional in that garnet is a usual accessory constituent in the gabbro. The development of secondary hornblende does not seem to be as prevalent in the gabbro of this area as it is in the gabbro of Maryland.