242 H. P. CUSHING—SYENITE-PORPHYRY DIKES. 
igneous activity which formed these dikes. The same is true of the ac- 
companying diabases, the exhibition of them here being a very impress- 
lve one, as was long ago noted by Emmons.* 
The remainder of the known dikes occur within a radius of less than 
20 miles from Rand hill, the one at Burke alone excepted. The latter 
is the only one yet found by the writer in Franklin county, though he 
has been over two-thirds of it, nor have Professors Kemp and Smyth 
found them as yet in Essex and Saint Lawrence counties. The available 
evidence indicates for them a quite restricted distribution, much more 
so than is the case with the diabases which abound in Essex and Frank- 
lin counties as well as in Clinton, and which outnumber many times the — 
porphyry dikes. 
GroLoGcic AGE 
In Ellenburgh township, Clinton county, are widespread exposures of 
a massive, basal conglomerate of Potsdam age. Its pebbles are mainly 
of quartz, derived from the quartz and pegmatite veins which abound in 
the gneisses of the vicinity. The matrix is of coarse sand, made up of 
quartz, orthoclase, and magnetite—in other words, of well disintegrated 
gneissic debris. These ledges furnished abundant material to the south- 
westwardly moving ice-sheet, and boulders are very common around 
lower Chateaugay lake, but a few miles away in that direction. Among 
these blocks two of unusual character were found by Professor A.C. Gill 
and the writer during the past summer. They are much coarser, show 
less water action, and consist of less thoroughly disintegrated material, 
fragments of gneiss and of dike rocks reaching several inches in diam- 
eter, embedded in finer material of the same sort.f Both of the boul- 
ders contained diabase pebbles { and one of them showed two consid- 
erable masses of syenite-porphyry.§ It must be frankly admitted that 
no similar conglomerate has been seen in place in the district, but its 
Potsdam age is regarded as certain, because no other unmetamorphosed 
conglomerate exists in northern New York except that at the base of the 
Potsdam, because this boulder has evidently not traveled far from its 
parent ledge and is associated with numerous blocks of undoubted 
Potsdam conglomerate, whose outcrops are not far distant, and because 
it is precisely the kind of rock which one would a priori expect to be 
* Nat. Hist. of New York, part iv, vol. ii, p. 25. 
+See R. Pumpelly: Bull. Geol. Soe. Am., vol. 2, pp. 209-222. This evidently represents a rock 
formed from material only partially disintegrated, after the more completely broken down, over- 
lying material had been removed from the old land surface by the waves of the encroaching Pots- 
dam sea. ‘ : 
{ Diabase pebbles have been found at several localities in Potsdam conglomerate in place. 
2 This boulder lies near the house of Mr Martin Shutts in Bellmont township, Franklin county, 
half a mile west of lower Chateaugay lake, It is deeply embedded in the soil. 
