IOWA ACADEMY OF SCIENCES. 25 



The proofs that the granites in question are eruptive in nature is deduced from 

 several different and independent sources: 



(1) From field relations. 



(2) From inclusions. 



(3) From contact phenomena. 



(4) From microscopical examinations. 



(1) Field relations of the granites. — As stated efsewhere, the eastern half of the 

 Piedmont region consists chiefly of gneisses broken through in numerous places 

 by undoubted eruptives, as gabbro, diorite, pyroxenite and others, until now these 

 rocks occupy fully one-half of the exposed surface of the district. Now, a careful 

 tracing of the granite shows that they have cut indiscriminately across the igne- 

 ous rocks mentioned, a^ well as the gneiss, passing uninterruptedly from one petro- 

 graphically distinct mass to another. In other words, the acidic types of crystal- 

 lines to all appearances seem to be younger in age than the gabbros and the most 

 basic rocks, as if they, too, had broken through all the other eruptives. Near some 

 of the granite masses true granitic and felsitic dykes are clearly defined and would 

 ordinarily be regarded as apophyses of the main body, were the rock regarded 

 as eruptive. Furthermore, at Dorsey's Run Station, for instance, large exposures 

 show the granite spreading widely apart enormous layers of twisted and puckered 

 gneiss. At Woodstock huge blocks are completely enclosed in the granite. 



As already remarked, the line of contact between the granite and the contigu- 

 ous rock is seldom determinable exactly on account of profound superficial decay. 

 Yet, occasionally artificial excavations into the acid rock reveals clearly such con- 

 tacts, as at the new quarry opened about two miles northwest of Garrett Park, 

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

 Eoapstone belt. 



(2) />ic?Hs/o»s.— Perhaps one of the most exclusive proofs of the eruptive nature 

 of some of the Maryland granites is the occurrence in the mass 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 the interior of the foreign 

 pieces are scarcely altered at all, though the exterior forms more or less com- 

 pletely metamorphosed shells of varying thickness. The Woodstock and Dorsey's 

 Run granites show similar phenomena equally well, or even better. In both 

 instances blocks of highly puckered gneiss are very prominent; and they all pos- 

 sess narrow marginal borders of dark, fine-grained, completely changed rock, 

 which contrasts sharply with the light colored, surrounding granite. Certain out- 

 crops near Garrett Park furnish good illustrations of the same kind; though 

 here the granite has been squeezed considerably more than in the other cases 

 mentioned. At this place there is one exposure showing numbers of small lentic- 

 ular masses of a black color, which might easily be taken for inclusions but for 

 their regularly bounded outlines. These are, doubtless, basic secretions which 

 developed in the acid magma. 



(3) Contact Phenomena. — For reasons elsewhere explained the contacts between 

 the granitic masses and the adjoining rocks are rarely seen to advantage. The 

 investigation of the contact zones have therefore been carried on largely with the 

 inclusions. This has been very satisfactory on account of the variety of foreign 

 rocks represented and the abundance of the fragments. In most of the fragments 



