REPORT OF THE DIRECTOR AND STATE GEOLOGIST 



rG3 



dikes have been described in considerable detail by Kemp. 1 

 They vary greatly in structure and mineral content. Practically 

 all are porphyritic, but in the center of the larger dikes the 

 porphyritic structure disappears, and the rock becomes a true dia- 

 base; otherwise it is more properly a diabase-porphyry. Olivine 

 is almost always present. 



These rocks have almost identically the composition of the 

 hyperite-gabbros, yet no garnet, so abundant in the gabbro, has 

 developed. On the other hand, biotite rims around magnetite, 

 specially at magnetite-feldspar contacts, are a conspicuous fea- 

 ture in the diabase, and occur also in the gabbro. The writer has 

 found no biotite in his freshest diabase dikes, and in those in 

 which it occurs it seems always to be surrounded by more or less 

 altered parts of what ma}' be otherwise quite fresh rocks. Hence 

 a disposition to regard it as secondary instead of primary and 

 also to regard it as produced in a more superficial zone than that 

 in which the garnet develops. 



The only rocks with which these diabases are likely to be con- 

 fused by anyone are some of the narrower dikes of hyperite-gab- 

 bro. From any of these they may be easily distinguished by their 

 finer grain, greater variation in grain from center to margin, 

 more flinty character and more excessive jointing. 



In only one case has it been possible to make a relative age 

 determination between these two sets of dikes. At the summit 

 of Rand hill, one fourth of a mile east of the house of L. San- 

 ger and on the east side of a knoll of gabbro, a 15 inch dike of 

 the syenite-porphyry, bearing n 65° e, is cut by a dike of dia- 

 base of the same width bearing east and west. In this case the 

 diabase is indisputably the younger. While this does not make 

 it safe to say that all diabase is younger than all the syenite, it 

 at least points in that direction. 



Paleozoic rocks 



Potsdam sandstone. Rocks classed as of Potsdam age are at 

 the surface over the larger part of the Mooers sheet. The rock 



x Kemp, J. F. Bui. 107 U. S. geol. sur. p. 24-29. 



