2 The American Geologist. July, isai 
Situated right in tlie midst of the Tacouic region Mt. (xre3'lock 
has been often alluded to during the last seventy years in the much 
dcliated "Taeonic Question." Professors C. Dewey, E. Emmons, 
E. Hitchcock and J. D. Dana are the principal authorities on the 
geolog}' of the mountain. The general sj'nclinal structure of the 
mass and also the fact that it consists mainly of certain schists 
underlaid l^}' limestone are well known. Professor Dana has also 
conjectured the anticlinal structure of the hollow which separates 
two of its ridges. 
The following description is based upon the new 20 ft. contour 
map made b}' the topographers of the U. S. Geological Survey, 
and upon extended and laborious geological explorations, and upon 
the careful microscopic stud}' of lithological specimens 1)}' Mr. J. 
E. Woltt". The results of modern topography, orography and 
petrography have Ijcen brought to bear upon the sul)ject. 
Structural The rocks are all metamorphic and of few kinds: 
crystalline limestones and various schists, micaceous (sericitic) 
chloritic, albitic, p3'ritiferous, plumbaginous, calcareous. The 
key to the real structure of the mountain is in clearly distinguish- 
ing cleavage-foliation from stratification-foliation, the apparent dip 
and strike being generally entirel}' misleading excepting at con- 
tacts and even there sometimes.* The phenomena of cleavage 
and stratification as the}' occur on Mt. Greylock are illustrated by 
a number of typical cases which substantiate and illustrate the 
following structural principles: 
I. Lamination in schist or limestone may be either stratifica- 
tion-foliation or cleavage-foliation or both. In rare instances traces 
of false-bedding occur in the limestone. To establish conforma- 
bility the conformability of the stratification-foliations must be 
shown. The importance of this is manifest and it would seem too 
elementary a principle to be stated here were it not that at one 
locality where the limestone and schist are in apparently conform- 
able contact, their cleavage-foliations alone are conformable while 
the stratification-foliation of the schist is at right angles to that of 
the limestone owing to a fault. 
II. Stratification-foliation in the schist is indicated by: (a) the 
course of minute but visible plications; (b) the course of micro- 
*The more important referonces to the subject of cleavage occur in 
the writings of Sedgwick, Phillips, Darwin, Sharpe, Sorby, Tyndall, 
Rogers, Ivycrulf, Uoim, Daubroe, Jannettaz, Reusch, Bonney, Cadcll and 
llarl<er. 
