596 



NOTES ON SERPEMTNOUS ROCKS — MERRILL. 



the limestone becomes a nearly pure rock, or free from intermixture with this sub- 

 stance. Whenever these two substances are commingled in the same mass, it is more 

 free from siliceous minerals either in the form of quartz, pyroxeue, or scapolite. It is 

 difficult to describe the rock in a few words as it occurs at Port Heury. It is pure 

 limestone near the furnace, quite coarse and crystalline. The steep rock west of the 

 public house is a mixture of yellowish serpentine and primary limestone ; * * * near 

 the dwelling of Mr. Foote, is a mixture of the same materials; the serpentine is 

 darker and the contrast between the limestone and serpentine is greater. * * * j u 

 the same bed, in addition to the mixture already mentioned, I found those of cocco- 

 lite and pyroxene in crystals, blood-red mica or mica which transmits a blood-red 

 light, hornblende, and limestone, etc. In the midst of the bed, half a mile from the 

 lake (Champlain), is an extensive one of calcareous spar. * * * It contains a great 

 abundance of graphite, etc. 



The typical ophiolite as put upon the market consists of a quite even 

 granular ad mi xt are of serpentine, calcite, and dolomite in particles 

 from one-eighth to one-fourth of an inch in diameter, interspersed with 

 small scales of phlogopite, occasionally graphite, and more abundant 

 pyrrohotite granules. As noted by Emmons, however, the texture is 

 variable, and, as seen by the writer at the now abandoned quarries near 

 the village of Port Henry, the frequent occurrence of large blotches 

 of yellow and greenish serpentine, or serpentine and white pyroxene, 

 in sizes from an inch to a foot or more in diameter, proved a serious 

 drawback to the production of marketable material. From the aban- 

 doned quarry of the "Ophite Marble Company," and other openings in 

 the vicinity, it is easy to obtain masses of the serpentine showing 

 pyroxenic nuclei, and all stages of the alteration are readily traced. 

 A.t the Ophite quarry was selected what seemed a typical sample, 

 (70085) and from it was picked out the unaltered pyroxene and the 

 secondary yellowish green serpentine. These, submitted to Mr. Cat- 

 lett, of the U. S. Geological Survey, for analysis, yielded as follows : 



The pyroxeue is therefore a very pure lime, magnesian variety, of 

 the formula CaMgSi 2 6 , and its conversion into serpentine consists, 

 as in the other cases described, in the assumption of water and giving 

 up its lime, which crystallizes out in the form of calcite. The resultant 

 serpentine is also of exceptional purity. The origin of the large 

 masses of the yellowish serpentine is thus readily accounted for. It is 

 to be noted, however, that the serpentine occurring in small particles 

 scattered evenly throughout the granular portion of the rock is of 



