144 G. H. Williams — Serpentine, etc. 



"green bands running perpendicular to the line of contact across 

 the entire surface. This green color is seen under the mi- 

 croscope to be due to the secondary development of a 

 green mineral in a very finely divided, state, which, to judge 

 from its color, extinction angle and pleochroism, is amphibole, 

 although it possesses no crystal form. More rarely there is 

 some brownish substance resembling mica, but not in a well 

 characterized form. Both of these are common contact miner- 

 erals in limestones and have here undoubtedly been secondarily 

 developed by heat. 



The main points of evidence, therefore, that the serpentine 

 at Syracuse was originally an igneous and intrusive rock, 

 belonging to the family of peridotites, are as follows: 



1st. The structure of the rock, which is such as is only known 

 to be produced by crystallization from a molten magma. 



2d. The existence of a more granular and a porphyritic mod- 

 ification, as is so often the case in eruptive dykes. 



3d. The inclusion in the rock of fragments of the adjacent 

 limestone and possibly of other rocks brought up from below. 



4th. The indication that these limestone fragments have been 

 modified by the action of heat. 



5th. The fact, stated by Mr. Wilkinson, that 50 feet away 

 from the exposure, on the strike of the rocks, only gypsum was 

 encountered. 



This evidence has been developed at such length, because 

 aside from its bearing upon Dr. Hunt's theory of the origin of 

 serpentine, this rock is interesting as being the only known 

 instance of an igneous intrusion in the unaltered and undis- 

 turbed palaeozoic strata of New York. 



In conclusion it is worth while to notice the extremely close 

 resemblance between the Syracuse rock and the peridotites of 

 Carboniferous age which Mr. J. S. Diller of the U. S. Geologi- 

 cal Survey, has recently described from Elliott Co., Ky.* These 

 rocks are intrusive through Carboniferous strata and hence 

 must be of as late origin as these, although they may be con- 

 siderably younger. They contain fragments of the adjoining 

 shale, which they have metamorphosed in a manner similar to 

 that in which the dolomite inclusions have been changed by the 

 Syracuse peridotite. Fragments of syenite and granulite were 

 also found analogous to those granitic and syenitic aggregates 

 mentioned by Vanuxem. In structure the two rocks present the 

 closest possible similarity. The size and form of the large por- 

 phyritic olivine crystals appear to 'be identical in both; the 

 groundmass of both also has the same appearance, containing 



* ''Science" for January 23, 1885, p. 65; this Journal, xxxii, p. 121, August 

 1886, and Bulletin of the TJ. S. Geological Survey, No. 38, the proof-sheets of 

 which were kindly placed at the writer's disposal by Mr. Diller. 



