LITHOLOGY. 211 



observation of exactly analogous structures in certain lavas and erupted 

 masses, which occurred in such circumstances that the origin of the 

 structure was plainly evident. To Mr. Scrope, gneiss is "squeezed gran- 

 ite." That mica will thus arrange itself under the influence of unequal 

 pressure has been confirmed by Mr. Sorby,* who submitted a mass of 

 pipe clay, in which he had mixed some micaceous hematite, to a press- 

 ure which extended the mass in one direction; and a subsequent exam- 

 ination proved that the scales of hematite had all arranged themselves 

 in planes at right angles to the pressure, producing a schistose structure, 

 and a more easy cleavage in the plane of the stratification. These exper- 

 iments have been of late repeated, with some unimportant variations, by 

 Mr. Daubree,t to show the effect of impeded movement. He forced a 

 plastic mass under great pressure through an aperture, and thereby de- 

 veloped a schistose structure, the laminae being parallel to the walls of 

 the opening ; and other results of the same nature were obtained. 



All these circumstances demonstrate that pressure which acts in such 

 a way as to produce lateral movements may be certainly effective in re- 

 ducing any foliated or tabular minerals in a plastic mass to an approxi- 

 mate plane ; and with our knowledge of the extent to which movements 

 have gone on in the crust of our earth, and which have accompanied the 

 elevation of mountains and contortion of strata, this circumstance fur- 

 nishes the most satisfactory explanation for most of the facts observed. 

 If the stratification of gneisses is an induced structure, then it does not 

 follow that this stratification should always correspond with the original 

 bedding; and the existence of eruptive masses of gneiss, in which the 

 structure cannot be referred to original sedimentary conditions, as has 

 been shown by Naumann,^ adds weight to the supposition. Mr. Charles 

 Darwin has also noticed the gneissoid structure in certain Chilian erup- 

 tive granites. Neither does it follow, that if the lamination of the gneiss 

 does correspond with the plane of the strata, the lamination is to be re- 

 ferred to the stratification of sediments, for the cleavage of some clay 

 slates, which is admitted to be due to pressure, very often corresponds 

 with the plane of the bedding. If a mass of granitic material is fused or 



*Sorby. Origin of Slaty Cleavage. Edinburg Philos. y<?«r., 1853, p. 144. 

 t Comtes Rendus, March 27, 1876. 



% Uber die wahrscheinlich eruptive Natur tnancher Gneisse und Gneisse-Granite. Prof. C. F. Naumann. 

 yahrbuch fur Min., 1847, p. 297. 



