While in the field in the northern Adirondacks during the past few years the writer has noted dikes from time to time which prove to possess 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 district 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.† The more acid dikes under level . . .