18 REPORT OF NATIONAL MUSEUM, 1889. 



same substance, the process having been arrested before completion. 

 The same condition of .affairs is shown in the smaller specimens num- 

 bered 39038, 39101, 39110, and 69195 in the case, several of these having 

 been cut and polished to better show the various stages. In many 

 instances the calcium set free has crystallized out by itself in the form 

 of calcite of a blue gray tint. The exteriors of many of the nodules, it 

 will be observed, are grooved and striated like glacial bowlders owing 

 to expansion and consequent crowding in the process of hydration. 

 (See original paper "On the Serpentine of Montville, New Jersey," in 

 the bound pamphlets on the table.) Serpentine after pyroxene is also 

 admirably shown in the large polished block from Thurman, Warren 

 County, New York. 



Serpentine after tremolite is shown in specimens 70131, from New 

 York, and in specimens 70114, 70115, 70119, 70121, and 70122, from 

 Easton, Pennsylvania. The least changed rock is shown in specimen 

 70L22, and the different stages of the alteration are indicated in gradual 

 change in color from light gray, nearly white, to greenish colors. 



Serpentine after a peridoti.te is shown in specimen 70137 from the 

 Lizard, Cornwall, England, and in the deep, bright green rock from near 

 Texas, Pennsylvania (701G0). Serpentine after pyroxenite is shown in 

 specimens 38478 and 38471 fron* Montgomery County, Pennsylvania, 

 the first mentioned being the fresh enstatite rock, while the last is the 

 impure serpeutiuous product to which it gives rise. Serpentine after 

 pyroxeue and replacing calcite, giving rise to the mixed serpentine, cal- 

 cite, and dolomite rock, ophiolite, is shown in specimens 70082 to 70084 

 from Moriah, Essex County, New York. 



From Plutonic or hypogeue, we pass to (n) Surface or epigene action, 

 in which section are displayed materials illustrative of the destructive, 

 constructive, and reproductive effects of the atmosphere, of surface 

 water, and of life in its various forms (Geikie p. 301 et seq.) 



AIR. 



Pure dry air, as stated by Professor Geikie, has but little effect upon 

 rock masses, and it is only through the aid of dissolved moisture and 

 temperature variations that its efficacy as a geological agent becomes 

 apparent. Nevertheless, there are certain phenomena which, although 

 it may be of slight geological importance, are sufficiently interesting to 

 fiud a place here. 



Effects of lightning. — Under this head are exhibited an interesting 

 and instructive series of fulgurites or lightning tubes. Specimens of 

 tubular form like those from Illinois (35905), New Jersey (39313), South 

 Caroliua (37414), Florida (38312 and 73263), and Maldonado, South 

 America (38852), are formed by the lightning striking in loose sand, 

 the heat of the flash being sufficient to fuse the sand, and thus form on 

 cooling the frail glassy tubes shown in the specimens. When lightning 

 strikes on solid rock the more common result is a mere superficial 



