4 PROCEEDINGS OF BROOKLYN MEETING. 



ON A BASIC BOCK DERIVED FROM GRANITE 

 BY C. H. SMYTH, JR. 



\_Ahstract\ 



Intimately associated with the hematite of the Old Sterling mine, Jefferson 

 county, New York, is a dark, rather massive rock, which was described by E. Em- 

 mons as serpentine and has always been referred to as such. Some portions of 

 the rock closely resemble serpentines, but most of it contains numerous irregular 

 grains of quartz. The field relations are those of intrusive rock, but the abundant 

 quartz indicates that it could hardly be of the basic character usually found in 

 rocks which alter to serpentine. The true nature of the original rock is shown by 

 a few limited areas which have suffered comparatively little change. These cores 

 of unchanged rock are but a few yards in extent and consist of a reddish coarse- 

 grained granite composed chiefly of quartz and feldspar. This granite, evidently 

 very acid, shades gradually into the surrounding dark colored rock of serpentinous 

 aspect. Under the microscope this change is seen to result from the gradual re- 

 placement of the feldspar, and to a less degree of quartz by an aggregate of minute 

 green or yellow scales. Every stage of the process is shown from a slight altera- 

 tion of feldspar to a complete replacement of all of the constituents of the granite 

 by the green aggregate. As to the mineralogic affinities of the aggregate it seems 

 more closely related to the chlorites than any other minerals, and is quite different 

 from serpentine. Chemical analysis of a completely altered specimen shows about 

 30 per cent of SiO.^, 12 per cent of H^O, 11 per cent of MgO, and 27 per cent of FeO. 

 These figures show a great amount of change from the original acid granite and 

 point to chemical rather than mechanical agents as the chief factor in the process. 

 The rockis considerably crushed, but it is probable that this acted chiefly in giv- 

 ing access to the solutions causing the alteration, as dynamo-metamorphism alone 

 does not usually change the bulk composition of a rock so greatly. From the large 

 amount of iron in the rock, together with the fact that similar rocks occur at all 

 the iron mines, but nowhere else in the vicinity, it seems reasonable to infer that 

 there is a causal connection between the iron ore and the altered granite. Several 

 facts indicate that the ore is a replacement of limestone, probably through the 

 action of solutions derived from oxidizing pyrites near at hand. Such solutions, 

 together wdth those resulting from the dissolving of the limestone, would afford 

 just the sort of agent required to effect the profound change shown in the granite. 



Remarks were made upon the paper by Jed Hotchkiss. 



The next paper vv^as — 



ASTUDYOF THE CHERTS OF MISSOURI 

 BY EDMUND O. HOVEY 



H. S. Williams and N. S. Shaler made remarks upon the matter of the 

 paper, which is printed in the American Journal of Science for Novem- 

 ber, 1894, voL xlviii, pp. 401-409. 



