CHEMISTRY. 217 



these layers. There is also, in one of my specimens, a plane, 

 forming an angle of 15 with the plane of readiest cleavage, and 

 perpendicular to the planes of the fissures. It appears to be a 

 joint, and is covered with small crystals of quartz. Scattered 

 through the rock are small scales of mica, which are compara- 

 tively few in number, but of uniform size, thin, and determine 

 the cleavage of the rock. These plates lie in parallel planes, as 

 they would settle naturally from a liquid. Where they are abun- 

 dant in any plane the rock splits there readily. If a piece, thus 

 split, be rubbed down perpendicularly to the cleavage plane, no 

 scales of mica are perceived upon this new surface, because the 

 observer is looking at the edges of the micaceous plates, but the 

 lamellar nature of the rock is thus made very apparent. 



*' The chief constituent of the rock under the microscope is 

 quartz in very sharply fractured and fine grains, together with a 

 little mica. Occasionally are to be seen very minute black grains. 

 .The specific gravity of the North Carolina specimen, taken in the 

 ordinary manner, is 2.61. The analysis yields the following re- 

 sults : 



Vol. at red heat (water) . . . . 0.17 



Silica ....... 95.89 



Sesquioxide of iron . . . . . 2.78 



Lime .... 0.84 



99.68 



" The relative hardness of the siliceous grains in the mineral ap- 

 pears to be that of agate, which may be scratched slightly with 

 them. On one occasion the bottom of an agate mortar received a 

 very decided scratch, which gave color to the supposition of a mi- 

 nute diamond as the cause. The rock is quite porous when com- 

 pared with other sandstones ; water placed in an excavation will 

 filter very readily through the stone, even in a direction perpen- 

 dicular to the plane of stratification. Gases diffuse very readily 

 through these pores. 



" The flexibility of this rock is attributed universally to the mica 

 which it contains, an inference which the microscope shows to be 

 unwarranted. This flexibility is due to small and innumerable 

 ball-and-socket joints, which exist throughout the mass of the 

 stone very uniformly. Each joint permits a slight movement, 

 which is always greater in one direction. The accumulation of 

 joints suffers a limited motion in every direction. This mobility 

 is not perceptible in thick slabs, unless they are large as to their 

 other two dimensions. This jointed structure is so wonderful that 

 it would warrant the name ' articulite ' to be given to the mineral. 



"Under the microscope, a fragment of the mineral is seen to 

 be composed of small botryoidal masses or congeries of grains of 

 sand. The observer can hardh' divest himself of the impression 

 that he sees a loose layer of sand, until the stage is again inverted 

 and jarred, which demonstrates that the grains are interlocked. 

 By managing the reflected light with which the object is illumin- 

 ated, the plates of mica can be distinctly seen ; they are very few 

 in number, and take no part whatever in determining the motion. 



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