44 The American Geologist. January, 1895 
which, for instance, an eruptive rock has solidified, the prob- 
lem becomes more and more difficult in proportion to the 
amount of change, \intil finally a- point is reached where it is 
absolutely impossible to say with certainty what the real na- 
ture of the stony mass was in the beginning. 
In the consideration of the wide-spread effects of regional 
metamorphism the agency of tangential pressure as the result 
of orographic movements is by no means the least important. 
Since the appearance of the classic work of Heimf the influ- 
ence exerted by this one factor has become more and more 
clearly understood, as may be inferred from the writings of 
Bonney,;J; Hatch, § Lehmann, || Reusch,^ Tiu-nebohm,** 
Schmidt,! f Teall, + + Williams§§ and others. 
But to return to the epidote belts in the plagioclase crystals. 
Dr. Grimsley also reinarks that the undoubted causal relation 
which exists between the zonal structure of the feldspars and 
their alteration whereby those zones richest in lime have been 
most completely changed to epidote, greatly favors the opin- 
ion advanced. This view also finds substantiation in other 
granitic areas and probably furnishes a key to the problem of 
why similar rocks through metamorphism and under appar- 
ently the same physical conditions change to very different 
masses. It is probably a principle of very wide application 
and one which will doubtless furnish a clew to many ques- 
tions concerning metamorphism which have been long re 
garded enigmatical. 
With the Port Deposit rock, which, like the Rowlandville 
variety, is in all probability of the eruptive origin, there is a 
very distinct foliated structure that has been produced sec. 
ondarily through pressure. The result of more intense dy- 
namic action has been to crush the minerals, thus giving rise 
to cataclastic rather than mineralogical changes, as epidotiza- 
fUntersuch. iiber den Mech. der Geb. u. s. w., Band II, 187S. 
:}:Quar. Ji)ur. Gool. Soc, London, vol. xlii, 1886. 
gTschermak's min. und pelrog. Mitth., Bd. VII, 1883. 
||Undersuch. ubcr die Ent. der altkry. Schiefergosteine, u. s. w. 1884. 
ITNeues Jahrbuch, BB. V, 1887. 
**Gool. For. Stockholm Boi-handl., Y, 1880. 
ffNeues Jahrbuch, BB. IV, 1886. 
tJGeological Magazine, Nov., 1886. 
§^U. S. Geol. Sur. Bui., No. 28, 1886; also ibid.. No. 62, 1890. 
