88 NEW YORK STATE MUSEUM 



interesting exposures appear in the cuts of the railway on the 

 shores of Lake Champlain,. one north of Craig harbor and the other 

 north of Cheever dock. The actual exposures of definite gabbro 

 are not of themselves extensive but their intimate association with 

 dark hornblendic gneisses has given rise to the view that the latter 

 were derived from the former. The possibility that the gneisses 

 may be basic syenites, to whose mineralogy they are more closely 

 related and that the gabbro may be intrusive in them has but 

 recently heen appreciated. It seems not unlikely that the horn- 

 blendic stringers, so prominent in the Grenville limestones, may be 

 derived from the syenitic rocks rather than the basic gabbros. 



Areal distribution of the basaltic dikes. The dikes are general 

 in their distribution and no part of the area can be said to be more 

 abundantly provided with them than is another. Our knowledge 

 of their occurrence is rather a function of extended exposure or of 

 artificial excavations such as mines, than of variations in distribu- 

 tion. Undoubtedly there are many more undiscovered. 



The one controlling feature in their structural relationships is 

 the development of lines of weakness and as the master lines run 

 northeast and southwest, it is along these that the basaltic magma 

 has chiefly broken through. The dikes have been observed cutting 

 each of. the more extended formations here described. 



Dikes of the same type of rock as those here described are 

 much more numerous to the north. Professor Gushing has found 

 them in great numbers in Clinton county and so far as we can 

 judge this area is probably over the focal source or was most 

 accessible from it. These dikes are known as far south as Fort 

 Ann, but they appear to decrease in number to the south. 



Chapter 8 



AREAL DISTRIBUTION AND GENERAL STRUCTURE OF THE 

 PALEOZOIC FORMATIONS 



The Paleozoic rocks outcrop on the New York side of the Port 

 Henry sheet in three separate areas, the largest of which extends 

 from Westport 7 miles to the south a little beyond Mullen brook. 

 The second is a little longer than 2 iniles forming the site of Port 

 Henry, while the third constitutes the Crown Point peninsula. The 

 latter is the northeastern continuation of the Crown Point area of 

 sedimentary rocks of the Ticonderoga sheet. These areas appear 

 as embayments in the eastern margin of the mass of the Adirondack 

 crystalline rocks and owe their preservation mainly to their being 



I 



