GEOLOGY. 305 



ing to an anticlinal axis at a few miles distance north and south. Hence 

 there has been a cap denuded, three-fifths of a mile thick. Doubtless, all 

 along this anticlinal axis, wherever the upper rocks have been removed, the 

 gneiss beneath will be discovered. It corresponds to the gneiss and horn- 

 blende slate of eastern Hampshire and western Worcester counties, and 

 would seem thus to lie below the Metamorphosed Silurian rocks. If so, it 

 should be included among the Hypozoic rocks. 



Sections similar to the foregoing have been measured and described by 

 English geologists. Not being aware of any similar work in our own coun- 

 try, I have described this section in hope of drawing attention to the subject, 

 and thus insure descriptions of sections far more grand and interesting. I 

 can vouch, on the part of the Vermont survey, for a careful attention to this 

 subject. 



ORIGIN OF SLATY CLEAVAGE. 



The view of Mr. Sharpe, presented to the Geological Society of London in 

 1487, that slaty cleavage is owing simply to pressure, and is at right angles 

 to the line of force, has been established by various facts brought forward 

 by Mr. J. Tyndall. (Phil. Mag. [4] xii. 35.) He shows that a fine clay, or 

 almost any impalpable material, when subjected to pressure and at the same 

 time allowed to spread laterally, takes a laminated structure. White lead, 

 wax, and even cheese, are among the substances which have afforded him 

 evidence on this point. He attributes the effect to the small inequalities 

 which exist in the texture of substances of all kinds. Under pressure the 

 mass yields and spreads out, and the little nodules become converted into 

 laminae, separated from each other by surfaces of weak cohesion, and thus 

 the mass becomes cleavable. The air cavities or fissures also spread out 

 thin under the pressure, and aid in producing the cleavage : for even dried 

 pipeclay shows such cavities in great numbers, many too small to be seen 

 without a magnifier. 



Mr. H. C. Sorb} r had before attributed the cleavage to the effect of such 

 lateral pressure on all the grains or pebbles in the rock ; the force making the 

 particles, whether of mica or any stones, to place their larger diameters in 

 the plane at right angles to the direction of the pressure, that is, in the plane 

 of cleavage. He had appealed to facts in the slates of Wales, showing that 

 where mica scales were present, they had this position. This cause may 

 contribute to produce lamination, though not appearing to be necessary to 

 the result. 



In the Philosophical Magazine for December last, Rev. S. Haughton gives 

 the results of some calculations to ascertain the amount of the pressure 

 which was exerted in cases of the production of slaty cleavage, using as 

 data the degree of distortion or compression of fossils. SiUiman's Journal. 



ON LAMINATION AND CLEAVAGE OCCASIONED BY THE MUTUAL 

 POSITION OF THE PARTICLES OF ROCKS WHILE IN IRREGULAR 

 MOTION. 



Mr. S. P. Scrope, in a recent communication to the London Geological 

 Society, on the above subject, referred to a former paper read by him before 

 the Society in April, 1856, in which this topic was touched upon, and pi*o- 

 posed to carry on the inquiry as to the probable effect, upon the internal 

 structure of rocks, of the mutual friction of their component parts when 



2G* 



