GEOLOGIC AGE. 243 
locally formed by a sea encroaching on an old and deeply disintegrated 
land surface as the Potsdam sea did. As therefore these dikes are only 
found cutting pre-Cambrian rocks and as they furnished material to the 
Potsdam formation they must be of pre-Potsdam age. 
' That they are younger than the pre-Cambrian gneisses and anortho- 
sites which constitute the mass of the Adirondacks is shown by the fact 
that they are found cutting both; but these old rocks have suffered 
profound dynamic metamorphism of such a character as to indicate 
that the rocks now exposed at the surface were then deeply buried be- 
neath rocks since eroded away. The greater part of this erosion took 
place before the deposition of the Potsdam sandstone. The dikes, how- 
ever, are unmetamorphosed and apparently did not cool at any ereat 
depth. Between the date of the anorthosite intrusion and of the depo- 
sition of the Potsdam sandstone there was a great time gap, during which 
the Adirondack region was above sealevel. The dikes are thought to 
have been formed toward the latter part of this interval. 
LITERATURE 
So far as the writer is aware, the only published reference to the rocks 
under discussion isin an article by A. S. Hakle, in which a dike, clearly 
pertaining to this group, is described from Upper Chateaugay lake.* 
The description, though short, is quite accurate, bringing out the differ- 
ence between these rocks and the bostonites.{ 
The writer has also referred to these rocks in a general way, calling 
them bostonites and orthoclase-syenite-porphyries, in two reports to Pro- 
fessor James Hall not yet published. 
MATERIALS COMPOSING THE DIKkEs 
MEGASCOPICAL APPEARANCE 
To the eye the rocks from the various dikes differ considerably. Nearly 
all of them show phenocrysts of feldspar, which may reach a leneth of 
one centimeter or even more, and which vary much in size and in abun- 
dance in the different dikes. In the case of two of the dikes they seem 
entirely lacking, while on the other hand they form in another case from 
10 to 15 per cent of the rock. They arecommonly of red color, but may 
* Am. Geologist, vol. xii, p. 31. ; 
{1am inclined to believe that Mr Eakle is in error in locating this dike on Indian point. He 
was possibly misinformed as to the name of the point on which he found it. Professor Gill and I, 
in company, searched Indian point carefully for this dike and were unable to discover it. On the 
other hand, the point is all cut up with branches of a large diabase dike of which Eakle makes no 
mention, though he describes two diabase dikes from the lake shore. This dike could by no pos- 
sibility have escaped his attention had he been on Indian point, 
