bo 
1896. | NEW YORK ACADEMY OF SCIENCES. 251 
The typical camptonites are apparently confined to the Paleo- 
zoic rocks. This peculiarity of distribution, coupled with the 
close agreement in chemical composition between the viabases 
and the camptonite sand with the fact that they seem to grade 
into one another lead Kemp to the idea that the difference be- 
tween the two were due mainly to variations in the physical 
conditions under which solidification took place.* Under the 
circumstances it becomes a difficult matter to decide whether 
any of the Pre-Cambrian, camptonite-like diabases should be 
classed with these later camptonites or not. We certainly should 
expect to find them, or their representatives, in the earlier rocks, 
unless we accept the notion that the diabases in toto are their 
representatives, thus precluding two periods of dike formation. 
Not only are the diabase dikes more abundant, but they are 
far more widespread than the others. As work is pushed west- 
ward into the Adirondacks, they are everywhere found. The 
other dikes seem however to have had their main centre of ac- 
tivity in western Vermont, and to decrease rapidly in a 
going north and west. 
Their Scarcity north of the Adirondacks.—In order to fully 
demonstrate the existence of a Pre-Cambrian period of dike 
formation in the region under discussion, it will be necessary to 
find dikes which are cut off by the Potsdam sandstone, such as 
have been recently described by Smyth} from the Thousand 
Islands. Such a case has not yet been met with. During the 
past field season the writer mapped the Potsdam—Pre-Cambrian 
boundary from Lake Champlain west to the Franklin county 
line, along the northern edge of the Adirondacks. Along the 
entire distance no actual contact between the two rock series 
was seen, though in several instances gneiss or gabbro and coarse 
basal conglomerates were found within a few feet of one another. 
Though in this respect the work was disappointing, it was during 
its progress that the idea of two periods of dike formation 
changed from a working hypothesis into a matter of conviction. 
The boundary here has a very different character from that along 
Lake Champlain. From the outlying gneissic ridges a high 
level plain slopes away toward the St. Lawrence valley, occupy- 
ing all the northern part of Clinton and Franklin counties and 
a great area in Canada, with nearly horizontal Potsdam sand- 
stone as the surface rock throughout, and with coarse basal con- 
glomerates invariably appearing in force as the contact is 
approched. Along the lake on the contrary there is a much 
faulted strip, the descent to which from the outlying ridges of 
* Bull. 107. U. S. Geol. Surv. p. 27. 
+C. H. Smyth, Jr., Trans. N. Y. Acad. Sci. Vol. XIII., pp. 209-214. 
