r26 NEW YORK STATE MUSEUM 



The original rock would seem to have been quite rich in olivin. 

 It is now wholly altered, mostly to chlorite, though in some of 

 the larger individuals patches of serpentine remain in the midst 

 of the chlorite, the whole combination showing distinct mesh 

 structure. 



The pyroxene is also wholly altered, and to a mixture of cal- 

 cite and chloritic aggregate. Some of the calcite is in crystals 

 of considerable size, and some of these present apparent 

 pyroxene outlines, as if in these cases the alteration had been 

 to calcite simply. Not a particle of unaltered pyroxene could 

 be detected. In many instances it is not possible to be sure 

 whether the original mineral was olivin or pyroxene, but in the 

 majority it is possible. There is considerable magnetite present 

 in minute crystals, but the amount is rather scant in comparison 

 with most diabases. 



The feldspars are of the long, lath shape, and the majority 

 show multiple twinning. The extinctions are, in general, not 

 over 15° but in two or three cases run up to 20°. This, may 

 indicate the presence of both andesin and labradorite, but in any 

 case the feldspar is not very basic. The alteration products of 

 the augite fill the interspaces between the feldspar laths, giving 

 an approximation to the ophitic structure, though by no means 

 well marked. 



The noncrystalline, nonamygdaloidal and nonporphyritic 

 character of this rock shows that it must have cooled under 

 different conditions from the others; and Professor Wood- 

 worth's paper indicates the likelihood of this, since a fault inter- 

 venes, which downthrows to the north, implying that the rock 

 on the south is from a deeper seated source. Professor Wood- 

 worth writes me that his idea is that one passes here from the 

 throat, with ball structure, to a conduit in which the lava cooled 

 under different conditions, but that the exposures are not suffi- 

 ciently good to permit of certainty in the matter. The sections 

 would seem to reinforce this interpretation. 



Correlation. Though the rock possesses no special petro- 

 graphic interest, being an ordinary rotten diabase, it is inter- 



