230 TRANSACTIONS OF THE [may 21, 
norites and anorthosites, seamed by an unusual number of 
Dikes, there being no less than sixty within a distance of two 
and one-half miles. Following the lead of Professors Kemp and 
Marsters, who visited the Essex shore, but not this Willsboro’ 
locality, in 1889-’90, and who have fully described similar dikes 
in their monograph of the region,* we may distinguish the 
dikes of the two townships as: 
(1.) Feldspathic porphyries or trachytes, called in the above 
monograph ‘ Bostonites,” which are light colored, creamy, 
brownish white or reddish, breaking with rough surface. Micro- 
scopically they show trachytic structure with a ground mass 
of lath-shaped feldspars, frequently with flow structure; the 
clearness with which this is shown varies with the rock. 
(2.) Quartz porphyry, or rhyolite, showing numerous well- 
developed hexagonal basal sections of quartz as phenocrysts. 
Structure micro-granitic, with few lath-shaped crystals. 
(3.) Lamprophyres; dark basic varieties, which under the 
microscope contain plagioclase, augite, hornblende and biotite ; 
and are divided into: 
a. Diabase dikes; holocrystalline, ophitic aggregates of 
plagioclase and augite, with biotite, olivine and occasionally 
a little hornblende; not differing essentially from effusive dia- 
base. 
b. Camptonites ; with preponderating hornblende or augite ; 
olivine frequently present and at times a little glass; dark 
silicates and often the feldspars idiomorphic. 
c. Fourchites; without olivine; very fine-grained. mostly 
pyroxene or hornblende in a glassy ground mass and very 
little plagioclase; often containing amygdaloidal fillings of 
calcite } to 4 inch across. 
In the Archean there are in several localities, large dikes, 
conspicuous by their darker color from the adjacent walls, but 
which microscopically are shown to be metamorphosed to horn- 
blendic norites or gabbros usually ,with reaction rims of garnet. 
Two great dikes of this class may be seen strikingly against the 
cliffs from Willsboro’ Bay. Another occurs on a hillside in the 
woods in southwestern Essex, 45 of the map. 
In the Utica shale on Cannon’s Point, Essex, an extensive 
intruded sheet or laccolite of bostonite is visible along shore 
for over 200 yards. It extends inland one-fourth of a mile. 
* J.F. Kemp anv V.F. Marsters: Trap Dikes in the Lake Champlain Region 
and the Neighboring Adirondacks; Trans. N. Y. Acad. Sci. XI. (1891): 13. 
: The Trap Dikes of the Lake Champlain Region (1893), Bull. 107, U. 
S. Geol. Surv. pp. 17-22. 
For origin of terms see M, HuntrerR anp H. RosensuscH: Ueber Monchiqnit, ein 
Camptonitisches Gangesteine aus der Gefolgschaft der Eleolith-Syenite. 
Tschermak’s Min. u. Petrog. Mitth., XI. (1890): 445. 
