﻿208 Intelligence and Miscellaneous Articles. 



often traversed, both parallel to and across the foliation, by veins 

 of red pegmatite : these are of a later period of consolidation than 

 the rest of the rock. The author concludes, from field-evidence, 

 that the grey facies of the gneiss results from assimilation of the 

 country-rock by an ascending magma ; while the red facies represents 

 the residual portion of the same magma. 



The special interest of the district, however, lies not so much in 

 the granite, as in a system of tachylytic veins which everywhere 

 intersects the granitic rocks. These veins range from a fraction of 

 an inch to 2 feet in thickness, but in the thicker veins there are 

 numerous inclusions of the country-rock. They are irregular in 

 form, thickness, and direction, and are due to the intrusion of a 

 basic magma which underlay the district. The author describes the 

 microscopic characters of these tachylytes, and comments on their 

 general glassy and cryptocrystalline nature, which he does not regard 

 as a result of chilling, but suspects is dependent upon the viscosity 

 of the basic magma. 



He brings forward evidence to prove that the position occupied 

 by the tachylyte is independent of tectonic features, but follows 

 directly from solution and corrosion of the granitic rocks by the 

 basic magma. 



XXIV. Intelligence and Miscellaneous Articles. 



PRESSURE OF RADIATION ON A RECEDING REFLECTOR : 

 CORRIGENDA. BY SIR J. LARMOR. 

 rpHE correct statement (Phil. Mag. Nov. 1914, pp. 706-707) is 

 -*■ that the sethereal momentum inside and the mechanical 

 forces sustained by the bodies inside are the result of the stress 

 transmitted across the boundary : the sign of the extra tangential 

 traction must therefore be changed. 



Also equation (A) makes 6' of opposite sign to d : to adapt it to 

 (B) the sign of its first term must thus be changed. 



Equation (B) means that for oblique total reflexion, as for 

 direct, the force intensity is altered in amplitude inversely as the 

 wave-length, and in no other respect. Thus the phenomena can 

 still be formulated under a simple scheme which expresses that the 

 radiation carries momentum with it, and on total reflexion the 

 amplitude of the fundamental displacement in the aether is 

 conserved*. Then perhaps the validity may be asserted beyond 

 the first power of v/a : but for an ordinary partial reflector, the 

 propagation is in part into matter whose molecular structure is 

 naturally too complex for such a scheme. 



* I observe now that in a clear and concise investigation in the 

 Phil. Mag. for December, conducted in terms of momentum alone in 

 Prof. Poynting's manner, and thus applicable only to propagation in free 

 space, Mr. T. Harris finds that the resultant thrust on the perfect receding 

 reflector is along the normal. This fact either may be regarded as a 

 simple geometrical consequence of the principle above stated, or else 

 may be formulated as an alternative principle. 



