156 
Wisconsin Academy of Sciences , Arts and Letters. 
ON SOME METAMORPHOSED ERUPTIVES IN THE 
CRYSTALLINE ROCKS OF MARYLAND. 
By WM. H. HOBBS. 
Recent studies of the crystalline rocks, while they leave us still in doubt 
as to the origin and much of the subsequent history of the so-called/tmda- 
mental complex (Urgeneiss or Grundgebirge of Germans), yet have been 
very successful in explaining satisfactorily many areas of crystalline schis¬ 
tose rocks by the metamorphism of sedimentary layers or eruptive masses. 
Thus many areas of rocks not to be distinguished in the hand specimen 
from schists which lie below the lowest fossiliferous horizons, have been 
shown to be metamorphosed sediments of Silurian, Devonian, or even 
younger ages. In Norway and in western New England, Silurian beds have 
been changed to crystalline schists. Schistose Devonian rocks occur in the 
Ardennes and Taunus. In northern Italy, at Carrara, the Trias is repre¬ 
sented by marble and in other localities crystalline schists are assigned to 
the Cretaceous. 
In seeking to analyze more carefully the process and find analogies with 
other metamorphosing agencies, the attention is directed at once to those 
processes which take place within the contact-zones of eruptive masses. 
Careful comparison shows that as concerns mineral decompositions and 
the atomic and molecular re-arrangements, the two processes yield re¬ 
markably similar results. The hydrous minerals generally disappear, 
certain mineral species disappear and are replaced by other species 
formed from their constituents, which are more stable under the conditions 
of the process. The more common metamorphic minerals are mica, 
garnet, staurolite, epidote, vesuvianite, wollastonite, and certain varieties 
of pyroxene and hornblende. Analyses in toto of the unaltered and met¬ 
amorphosed phases of the same rock yield identical results, exceptions 
being noted when tourmaline, topaz, or allied species, in whose formation 
gaseous material is concerned, have been developed during the process. 
Mineralogically, then, the process consists in a more or less thorough re¬ 
arrangement, which may be mainly molecular as in the case - of hornblende 
after augite, but it more frequently involves elaborate chemical decomposi¬ 
tions and reactions. The German expression, Umbildung, aptly describe* 
this process. 
If the agent be sought which affects the transformations, we are directed 
to a different source in each of the two cases of Contact and Regional 
