Granites of Cecil County, in Xorth-eastern Maryland. 65 



mentioned, are to be regarded as basic segregations rather 

 than inclusions of foreign rocks, they must, by no means, 

 be confused with the finely-developed and abundant inclu- 

 sions of darker rock which occur in the granite along the gab- 

 bro contact. As one follows the granite from Rowlandville to 

 Porter's Bridge, over the fine exposures along Octoraro Creek, 

 a very typical eruptive contact is seen. After half of this 

 distance has been traversed, large and irregularly-shaped 

 fragments of a more basic rock begin to appear in the granite, 

 although these by no means possess the petrographical char- 

 acter of gabbro. These fragments, which are often seen to 

 be disrupted and traversed by the enclosing granite, become 

 steadily more abundant toward Porter's Bridge, while the 

 granite itself, as has already been stated, grows more basic. 

 The inclusions come to make up more than half the volume 

 of the rock, and, finally, at Porter's Bridge, the granite is 

 seen only to form intrusions into the more basic mass. With 

 our present knowledge, it is, perhaps, impossible to satisfac- 

 torily interpret these inclusions. They seem not to represent 

 the gabbro, at least in its typical development, but seem to 

 have some genetic relation to the granite itself. It is not im- 

 probable that they may represent more basic portions of the 

 granitic magma which have become differentiated and were 

 the first to solidify, and which were subsequently broken into, 

 brecciated, and included by later and more acid portions of 

 the granitic magma. These inclusions would thus corre- 

 spond to the schlieren of Reyer,* brecciated and included by 

 later intrusions (Nachschube), as he has described them in 

 the Tyrol and in the Sierra Nevada. The relations here en- 

 countered also resemble those found by Broggert between 

 the laurvikite and ditroite on Lange Sund, in Southern Norway- 

 A contact of this kind indicates that there is no very great 

 difference in age between the more acid and more basic rocks, 

 but that they represent different phases in the solidification 

 of the same mass. The whole eruptive contact, so well dis- 

 played along Octoraro Creek, forms, itself, a kind of transi- 

 tion from the granite to gabbro, and plainly indicates that the 

 latter is the older of the two rocks. 



"Theoretische Geologie, p. 545, 1888. 



fGroth's Zeits fur Krystallog, u. s. w., Vol. XVI, p. 106, 1890. 



