240 H. P. CUSHING—-SYENITE-PORPHYRY DIKES: 
considerable interest. At first they were correlated with the bostonites 
which cut the rocks along the shores of lake Champlain a few miles to the 
southeast.* Later, doubts crept in as to the correctness of this reference, 
due partly to certain differences in the rocks themselves which were found 
to be constant, and partly to the fact that the dikes under consideration 
were only found cutting the pre-Cambrian rocks and never the adjacent 
Potsdam sandstone, whereas the bostonites cut all the rocks of the dis- 
trict up to and including the Utica slate. In the case of the associated 
diabase dikes, which are very numerous, this peculiarity of distribution 
soon amounted to a demonstration of their pre-Cambrian age.f The 
more acid dikes under discussion, however, are by no means abundant, 
_and have a more restricted distribution than the diabases, so far as pres- 
ent knowledge goes. An opinion that they were also of pre-Cambrian 
age was hesitatingly expressed in a report to Professor James Hall, trans- 
mitted a year ago, but not yet published. A fortunate discovery in the 
field the past summer demonstrates the correctness of that opinion and 
in so much expands our knowledge of the volcanic history of the Adi- 
rondack region. 
GEOLOGIC OCCURRENCE 
So far as yet known, these rocks occur only in dikes. All traces of sur- 
face flows, if such occurred, have been removed by erosion. To venture 
an opinion as to the depth below the surface at which the present surface 
exposures were formed is an hazardous proceeding. Even the narrowest 
dikes are completely holocrystalline, yet the walls have always exerted 
a marked chilling influence, and the groundmass of even the largest 
dikes is rather fine grained. It is thought probable that the depth must 
be measured in hundreds of feet, but that it was by no means excessive. 
The dikes vary in size from 1 foot to over 30 feet in width, and their 
walls have a near approach to verticality. In common with the diabases 
and the post-Utica dikes they havea prevailing east and west trend, rang- 
ing from north 55° east to south 75° east. 
GrOGRAPHIC DiIsTRIBUTION 
The location of the known dikes is indicated on the accompanying 
map, figure 1, of Clinton county, New York, which shows them all with 
the exception of the large dike at Burke, Franklin county. They occur 
in greatest abundance on Rand hill, in Beekmantown and Altona town- 
ships. ‘he hill is an outlier of pre-Cambrian rocks projecting above the 
* Kemp and Marsters: Bull. 107, U. S. Geological Survey, pp. 18-23. 
7 H. P. Cushing: Trans. N. Y. Acad. Scei., vol. xv, pp. 248-252. 
