6o NEW YORK STATE MUSEUM 



ably represent the last basic dregs from the great center of erup- 

 tion which gave rise to the larger masses. In all cases noted they 

 are massive and unmetamorphosed except from weathering. Un- 

 doubtedly they followed the great period of metamorphism but 

 shared in some faulting. 



Under the microscope the dikes are distinctly basaltic in their 

 mineralogy. Plagioclase and augite are the chief minerals. Olivine 

 occasionally appears, and magnetite is of course in every slide. In 

 the thicker dikes and at the centers of those of moderate width 

 the texture is diabasic; that is, the feldspars are long and narrow 

 and well bounded. They form an interlacing network in whose 

 interstices are the dark silicates. Toward the borders, however, the 

 texture becomes porphyritic with a finer and finer ground mass 

 until at the border the ground mass is a dense, black glass. The dike 

 is cemented at times so tightly to the wall rock, or fused into it, 

 that it breaks more readily elsewhere than along the contact. In 

 several instances where cliffs are exposed along the course of a 

 dike, fragments of the latter may be seen, still adhering tightly to 

 the older walls, although the major part of the dike has disin- 

 tegrated and disappeared. These relations appear at the dike shown 

 just west of the highway and 2 miles south of Elizabethtown, and 

 also in the one on the northwest corner of New pond. 



No analyses have been prepared of the dikes within this area, 

 but a selection is here given of those which have been published 

 from neighboring localities. They will also be found compiled pre- 

 cisely as here given in New York State Museum Bulletin 95, 

 page 350. 



1,2 3 4 5 



SiOz 43-41 44.51 45.46 46.73 50.89 



AI2O3 19-42 19-99 19-94 16.66 15-39 



Fe.03 5-72; . (2-561 



FeO 6.69I ^-^^ '^-^^ \8.45J ^-^^ 



MgO • 5-9S 8. II 2.95 8.12 7.60 



CaO 9-11 8.15 S.32 8.03 8.75 



NasO •• 4-39 5-24 2.12 3.73 5.67 



1 Diabase summit of Mt. Marcy, Essex co. A. R. Leeds. N; Y. State 

 Mus. 30th An. Rep't, p. 102. 



2 Diabase. Upper Chateaugay lake, Clinton co. A. S. Eakle. Am, 

 Geol. July 1893. p. 3S. 



3 Diabase. Palmer hill near Ausable Forks, Clinton co. U. S. Geol. 

 Sur. Bui. 107. p. 26. 



4 Olivine diabase. Belmont township, Franklin co., dike 13. Analyzed 

 by E. W. Morley for H. P. Cushing. N. Y. State Geol. i8th An. Rep't, 

 p. 120; and 20th An. Rep't, p. r79. 



5 Olivine diabase Upper Chateaugay lake, Clinton co. A. S. Eakle, as 

 under no. 2. 



