288 FIELD COLUMBIAN MusSEUM—GEOLOGY, VOL. I. 
to the edge of that piece. This portion of the plane evidently was 
not sufficiently developed as a division plane to produce disruption 
of the piece when the meteorite struck the earth. 
That the three planes described represent a structure which 
existed in the meteorite before its entry into the earth’s atmosphere 
there can be little doubt. They are too regular to make it possible 
to consider them planes arising from fracture by shock and there 
are several other lines of evidence pointing to their preterrestrial 
existence. The most important of these is that their surfaces are 
slickensided. The slickensided character of the surface resembles 
that seen in terrestrial rocks and is illustrated in Fig. 2. It isa 
smooth, shining, somewhat undulatory, like a roche moutonnée surface, 
and bears short stria which on the same surface run in one general 
direction, but take different directions on the three several planes. 
These several directions are indicated in Plate XLIV, Fig. 2, where 
one of the fragments is represented as removed. ‘The color of the 
slickensided surfaces is somewhat darker than that of the crust of the 
meteorite, but there is no evidence of special heat having been 
developed by the force which produced the slickensides. This I have 
tested by cutting sections at right angles to the surfaces. The outlines. 
of the individual grains were found to be sharp and unaltered up to 
the slickensided edge. 
Since slickensided surfaces on terrestrial rocks are so far as 
known produced by slow differential movement in the mass under 
considerable pressure and while in the solid state, they may in the 
absence of any evidence to the contrary be assigned to the same cause 
in this meteorite. The conclusion seems fair therefore that these 
planes and surfaces were formed during the preterrestrial existence of 
the mass and that the mass must have been solid in its nature while 
in space. The three planes which I have described seem to me to 
resemble the joint planes of terrestrial rocks more than anything else 
I can think of and give us grounds for asserting the existence of joint 
structure in the rocks of space. Ido not know that well marked joint 
structure has been observed in any other meteorites except that noted 
by Meunier in one of the stones of L’Aigle*. This stone he regarded 
as possessing a joint fissure, but it was not as well developed as the 
planes of the Long Island stone. 
If the occurrence of joint structure in the Long Island stone is 
deemed proved, it is significant as pointing to a considerable mass. 
possessed by the body in space. Joint blocks of such size as this 
would not be likely to be developed in a small body. 

*Fremy’s Encyc. Chimique, Tome II., Meteorites, p. 457. 
