CHARACTER OF THE INCLUSIONS. 303 



where the line is very sharply defined between the granite mass and the 

 adjoining soapstone belt. 



b. Their Inclusions. — Perhaps one of the most conclusive proofs of the 

 eruptive nature of some of the Maryland granites is the occurrence in 

 the masses of large numbers of inclusions — fragments of foreign rocks, 

 both sedimentary and eruptive. These have all been described more or 

 less at length in another place, to which reference may be made for fuller 

 details. At Sykesville, where they occur so abundantly, the irregular 

 angular fragments and blocks of all sizes are identical with rocks in the 

 neighborhood. 



In most of the cases observed, the interiors of the foreign pieces are 

 scarcely altered at all, though the exterior forms more or less completely 

 metamorphosed shells of varying thickness. The Woodstock and 

 Dorseys Run granites show similar phenomena equally well or even 

 better. In both instances, blocks of highly puckered gneiss are very 

 prominent, and they all possess narrow marginal borders of dark, fine- 

 grained, completely changed rock, which contrasts sharply with the light- 

 colored surrounding granite. 



Certain outcrops observed in the vicinity of Garrett park furnish 

 good illustrations of the same kind, though here the granite has been 

 squeezed considerably more than in the other ease mentioned. At this 

 place there is one exposure showing numbers of small lenticular masses 

 of a black color which might easily be taken for inclusions but for their 

 regularly rounded outlines. These are doubtless basic secretions which 

 developed in the acid magma. 



c. Their contact Phenomena. — For reasons elsewhere explained, the 

 contacts between the granitic masses and the adjoining rocks are rareby 

 seen to advantage. The invest igation < >f the contact zones have therefore 

 been carried on largely with the inclusions. This lias been very satis- 

 factory, on account of the variety of foreign rocks represented and the 

 abundance of the fragments. In most of the fragments it is only the 

 outside which is changed to the depth of from two to four centimeters or 

 more, the interior still often preserving the rock in its original character. 

 so that no doubt arises concerning its composition and structure previous 

 to its embedding in the granite. 



The contact zones are in all respects identical with the contact belts 

 of other localities where acid eruptives have pushed up against the same 

 kind of rocks. 



Chemical analyses of the unaltered inclusions, the metamorphosed 

 shells, and the surrounding granites show that the altered shells have an 

 acidity intermediate between the inclusions and the granites. 



