326 C. II. Smyth, Jr. — Peridotite in Central New York. 



large percentage of lime is partly derived from the material of 

 the adjacent vein, the dike having been broken by movements 

 subsequent to its cooling, and re-cemented by infiltration. That 

 there have been several such movements along the fault plane 

 is shown by a series of slickensided vein surfaces, each slicken- 

 side having been covered by a layer of vein stuff, thus afford- 

 ing a smooth surface for the recording of the next movement. 



The foregoing evidence, microscopic and chemical, is suffi- 

 cient to class the rock of the Manheim dike as a member of 

 the peridotite group, and of that division of the group to 

 which Lewis* has given the name kimberlite. It is a well 

 known fact that the peridotites are, of all igneous rocks, the 

 least prone to produce contact metamorphism, and when to 

 this is added the fact of the extreme narrowness of the dike, 

 it is not surprising that no clear evidence is given of any effect 

 upon the enclosing rocks. Diligent search was made for frag- 

 ments of these rocks included in the dike, but none were found. 



Through the kindness of Professors G. H. Williams and J. 

 F. Kemp, the writer has been enabled to compare sections of 

 the Manheim rock with sections of the peridotites of Syracuse, 

 Ithaca, Elliot Co., Ky.,f anc l Pike Co., Arkansas.^: The Man- 

 heim rock differs from that of Syracuse in having magnetite 

 instead of chromite and in the absence of pyroxene as an 

 essential constituent. In the former respect it resembles the 

 Arkansas rock, and in the latter, the Kentucky rock. In hav- 

 ing olivine in the ground mass the Manheim rock differs from 

 all the others. But in spite of these slight variations, the five 

 rocks are closely related. It is interesting to note the unfail- 

 ing presence of perofskite in the rocks of these widely separa- 

 ted localities. 



As regards the age of the Manheim dike, it is certainly 

 younger than the Utica slate, and its mode of occurrence 

 is such as to indicate, with some degree of probability, the 

 time of intrusion. The fault in which the dike occurs is 

 one of a series of eight or more parallel faults, beginning near 

 Amsterdam on the east and occurring at irregular intervals 

 from there to Little Falls. These faults affect the rocks from 

 the Archsean gneisses through the Utica slate. The throw is 

 not large and varies a good deal, but the strike is always north, 

 to northeast. The simplest explanation of these faults is that 

 offered by Yanuxem fifty years ago, that they mark the west- 

 ern extension of the forces of the Appalachian uplift. If this 

 be true, it fixes the intrusion at the close of the Carboniferous, 

 unless it be assumed that the fissure was not filled when first 

 formed, which seems highly improbable. Periods of oro- 



* H. C. Lewis: The Genesis of the Diamond, Science, viii. p. 345. 

 f J. S. Diller: Peridotite of Elliot Co. Ky.. Bulletin 38, U.S. G. S. 

 % J. C. Branner, R. X. Brackett: The Peridotite of Pike Co., Ark., this Journal, 

 Julv, 1S89. p. 50. 



