County is still a matter of some uncertainty. Several diabase dikes break through the gabbro and are therefore certainly younger than the gabbro. At present there is no conclusive evidence to show that they are younger than the granite or the serpentine. More work in this region may reveal the presence of diabase dikes cutting the granite and the ultra-basic rocks. Both Leonard and Bascom have considered the diabase dikes of Cecil County to be continuations of the great Triassic dike which Lewis48 traced through eastern Pennsylvania as far as the Maryland border. Certain characteristics distinguish the diabase of Harford County from that of Cecil County. In the specimens of diabase described from Cecil County both the augite and plagioclase are comparatively unaltered, the development of secondary hornblende and epidote being noted only in a few instances. In Harford County, however, unaltered pyroxene is rarely found and the alteration of plagioclase to epidote is very common. Secondary hornblende and epidote are the usual constituents of the diabase. It is evident that the diabase of Harford County has undergone much more metamorphism than has that of Cecil County. This does not, of course, prove that the Cecil County diabase is younger than the Harford County diabase but it is an indication that such may be the case. Diabase dikes have often been noted in other areas as intrusive into gabbros, apparently occurring as later off shoots of the parent magma. Loughlin in his investigation of the gabbros and associated rocks at Preston, Connecticut, has cited the occurrence of diabase dikes very similiar to those of Harford County in which the pyroxene has been altered to uralite and the feldspar altered to saussurite. These dikes have broken through the gabbro and Loughlin concludes that the upper portion of the main gabbro magma consolidated and became ruptured while the lower portion was still molten. The lower portion then welled upward into the net-work of fissures, forming dikes. Diabase of two district ages has been described in eastern Pennsylvania,49 one pre-Cambrian and the other Triassic. The pre-Cambrian diabase is characterized by considerable "Lewis, H.C., Proc. Am. Phios. Soc., vol. lxxii, 1885, pp. 438-456. alteration; augite has been altered to chlorite, and feldspar to saussurite. This diabase has been called pre-Cambrian because it has undergone considerable alteration and because it is intrusive only into pre-Cambrian rocks among which gabbro of apparently the same age as the Harford County gabbro is included. The other type of diabase is intrusive into Triassic shales and shows none of the alteration effects so characteristic of the pre-Cambrian diabase. It is quite probable that the diabase of Harford County is also much older than the Triassic diabase of Cecil County and the areas farther north. These older dikes may in fact be off-shoots of the same parent magma from which the gabbro came. Any decision as to the true age of the gabbro and related intrusives of Harford County must rest in large part upon the age of the mica schists and phyllites which border these rocks on the north and west and into which dikes and stocks of the ultra-basics have been intruded. If these schists and phyllites are to be correlated with the Wissahickon mica gneiss of eastern Pennsylvania which Bliss and Jonas have found to be pre-Cambrian in age, then it is very probable that the intrusives also are pre-Cambrian. The meta-gabbro of eastern Pennsylvania has been called pre-Cambrian by these authors and there seems to be little doubt that this rock and the gabbros of Maryland are part of the same great intrusion. A study of the field relations and associations of the different eruptive types-granite, gabbro, and ultra-basic rocks-at once suggests that they all originated from a common source. Williams was the first to propose such an origin for the eruptive series in the neighborhood of Baltimore. Referring to this series he says:-"While this wide range of petrographic types can not be positively shown to have originated from a single magma, their relationships in the field are so intimate and the instances of the gradual passage from one to the other are so common, that this seems not to be an improbable hypothesis." Leonard considered the eruptives of Cecil County to be differentiation products of a single parent magma. Bascom also thinks it highly probable that the igneous rocks of Cecil County are the differentiation prod ucts of a single magma, differentiation taking place both before and after intrusion. The conclusion that these types were derived from a single parent magma through differentiation before and after intrusion seems to be the one most probable for the eruptive series of the Maryland Piedmont Plateau. Examples of series of eruptive rocks, differing considerably in composition in each member, but nevertheless considered as having been derived from a common parent magma, have often been cited in the literature. Notable examples are the Cortlandt Series and the igenous complex of Garabal Hill in the Southern Highlands of Scotland.50 SUMMARY 1. In Harford County, Maryland, gabbro has been intruded into the ancient, highly crystalline schists and gneisses. The gabbro has the shape of a belt reaching almost all the way across the county but only three and a half miles wide at its broadest part. Its shape and the structure of the gneisses into which it has been intruded suggest that the gabbro may be in the form of a sill or sheet with a dip to the southeast. 2. For purposes of description two predominant varieties of gabbro have been recognized in this area, hypersthene gabbro and meta-gabbro. Study of the two types shows, however, that the meta-gabbro is only an alteration product of the hypersthene gabbro, due to the action of regional metamorphism. These two varieties grade into each other almost imperceptibly. 3. With the exception of a few small areas the whole of the unaltered gabbro is a hypersthene gabbro in composition, almost always containing the same mineral constituents. The relative amounts of these mineral constituents vary widely and abruptly, however. The grain of the rock also varies abruptly. These variations are thought to be due to change in physical conditions within the solidifying magma. 4. Near the contact of the gabbro with the Baltimore gneiss are areas 50 Dakyns, J. R. and J. J. H. Teall, Quart. Jour. Geol. Soc. London, vol. xlviii, 1892, pp. 104-121. which contain quartz epi-diorites and quartz-mica epi-diorites with a slightly coarser grain and a different mineral composition from the main mass of the gabbro. Field relations and microscopic study of these rocks indicate that they have been formed by assimilation of some of the adjacent Baltimore gneiss. They probably solidified somewhat later than the main mass of the gabbro. 5. Augen-gneisses are often associated with these epi-diorites. Their structure and mineral content indicate that the augen-gneisses have been derived from the Baltimore gneiss principally through contact action of the gabbro. The heat of the intrusive gabbro magma has increased the activity of solutions within the gneiss, causing feldspar eyes to be deposited and perhaps increasing the formation of chlorite. Pressure and heat from the gabbro have also caused kneading and crinkling of the gneisses. 6. The gabbro belt of this county is probably a part of the same intrusion as the gabbros, hypersthene gabbros, and norites of eastern Pennsylvania, Delaware, Baltimore County and Cecil County. At present, with the scarcity of evidence in Harford County and adjoining regions, it is impossible to determine exactly the age of the gabbros and associated intrusives. If the schists of the northern part of the county which have been invaded by the serpentine are to be correlated with the Wissahickon mica gneiss, then it seems probable that the age of the intrusives is pre-Cambrian. 7. The gabbro shows quite clearly its age relative to the associated igneous formations-granite, altered peridotites and pyroxenites, and diabase. The gabbro is undoubtedly younger than the Baltimore gneiss into which it is intrusive. The granite, however, is younger than the gabbro, as is indicated by their field relations in Harford County and Baltimore County. The serpentines, altered peridotites and pyroxenites, are also younger than the gabbro, although their age relative to the granite is not indicated by any evidence so far available in Harford County. The diabase is certainly younger than the gabbro, while evidence from other sections of the Piedmont Plateau indicates that it is probably also younger than the other igneous formations. Differences in degree of alteration in this diabase and in the diabase of Cecil County seem to show that the diabase of Harford County was formed at an earlier date. It may be closely related in time to the other igneous intrusions. 8. Evidence from this and adjoining areas makes it seem very probable that all the intrusive bodies except those which form portions of the Baltimore gneiss are products of a common parent magma, considerable differentiation having taken place in the parent magma before intrusions of the present igneous formations began. 9. Regional metamorphism and the activity of solutions which often accompanies that force have profoundly altered all the intrusive rocks. of the region as well as the sedimentaries. Peridotites and pyroxenites have been converted to serpentine; gabbro has been altered to metagabbro, with change of pyroxene to hornblende and feldspar to saussurite; granite has become somewhat foliated; and diabase has been uralitized. |