Giant dike swarms, often hundreds of kilometers long, have produced flood basalts in large igneous provinces since the early Proterozoic. Dike patterns described as radiating from a central source are actually syntectonic swarms that curve and diverge according to lithospheric stress regimes, but they are similar in origin to smaller swarms with parallel dikes. Giant radiating patterns of dikes do not characterize most hotspots or large igneous provinces, and they are not always linked to crustal uplift swells. These mafic intrusions and the fractures they follow are essentially features of plate tectonics, not products of indeterminable deep mantle plumes. As a compelling example, the Early Jurassic central Atlantic magmatic province and its associated Pangaean rift zone are evidential products of subducted materials and convection in the upper mantle beneath the insulating Pangaean plate. Giant dike swarms were formed along lithospheric structures through plate tectonics, not by a coincidental deep mantle plume.

Complex swarms of mafic dikes extend across New England and Atlantic Canada. Radiometric dates, distributions and structural patterns, petrologic correlations, and geochemical analyses of the postmetamorphic dikes and related plutonic complexes are used to distinguish at least four petrogenetic groups, or igneous provinces, that span the Mesozoic. These groups are as follows. (1) The Early Cretaceous New England—Quebec igneous province, which contains thousands of lamprophyric dikes and at least 20 associated plutonic complexes in northern New England and southern Quebec. (2) The Early to Middle Jurassic White Mountain magma series in central and northern New Hampshire, which contains numerous alkalic diabase and lamprophyre dikes associated with large syenitoid-granitoid plutons. (3) Quartz tholeiitic dolerites of the Early Jurassic Eastern North America dike province, occur as scattered, large dikes that also fed flood basalts, remnants of which are found in the Mesozoic basins. (4) Olivine diabase dikes are present along southeastern coastal New England (CNE groups), for which K-Ar dates and an association with the Agamenticus complex in Maine indicate Triassic ages. The CNE dikes may be correlated with Triassic intrusions in parts of Atlantic Canada (AC groups) as well as with undated dikes in southern New England.

Location Port Henry is a small incorporated village on Lake Champlain, within the Town of Moriah in Essex County of northeastern New York. The Craig Harbor site is at approximately 44°03' 30„N and 73°27'08„ W, within the Port Henry 15-minute Quadrangle (Fig. 1). The major state highway through Port Henry is New York 9N/22, but the village can also be reachedfrom I-87 (the Northway) off Exit 31 onto 9N south from Westport, or from Exit 30 east on County Road 6 to Mineville, southon County Road 7 to Moriah Center, and southeast on County Road 4 to Port Henry. The Beach Road turns eastward off Main Street at the baseof the fault-line bluff on which the village is situated, and parkingis available at a public boat launch and picnic area to the northof the road (Fig. 1). Walk north through the Villez Marina (alocal landmark facility!) and up onto the adjacent Delaware and Hudson railway. Exposures of the site extend north about 1,650 ft (500 m) to Craig Harbor, but excellent exposures also continuefor many kilometers along the railway and lakeshore. Stay off thetracks themselves, as approaching trains are difficult to heararound bends in the railroad cuts and may appear suddenly.