LAMINATED PYROCRYSTALLINE ROCKS. 
99 
cular force from the effects produced; it is a modified crystal¬ 
line force. 
In sandstones and limestones molecular movements often 
obliterate the planes of stratification or deposition. These 
movements result in the formation of spheroids, or the forms 
represented in fig. 17; a general illustration of the kind 
of molecular movement, which may be observed in many sedi¬ 
mentary rocks, and also in rocks of igneous origin, as serpen¬ 
tine. 
Fig. 17. 
We may recognize, too, in this phenomenon, one of the efficient 
causes of metamorphism, a cause which whenever the spaces 
between the particles of a mass are charged with water, or 
possess from any other cause a partial fluidity, is free to operate. 
The importance of recognizing the essential properties of 
matter as efficient causes of change in rocks, it seems to me 
has not been perceived, and hence has not been investigated so 
fully as it should be. Having stated the foregoing views re¬ 
specting cleavage planes or lamination, I proceed to speak of 
the characters of the laminated rocks, gneiss, mica slate, &c. 
§ 69. Gneiss. The rocks of this class, in whatever part of 
the globe they occur, are all alike and undistinguishable. A 
mass of gneiss from the Alps or Pyrenees, can not be distin¬ 
guished from a mass from the Alleghanies. The mica slate of 
the Alleghanies differs, in no respect, from the mica slate of the 
Rocky mountains. The same remarks may be extended to 
granite, sienite, and indeed to all the eruptive rocks. Fire has 
left the same impress upon all of them, in all quarters of the 
