Notes and Comments. 147 



tion of dibsolved material within a rock-mass. The Address 

 proceeded to the discussion of what is the most fundamental 

 characteristic of metamorphism : namely, that recrystallization 

 takes place in a solid environment, and so may be profoundly 

 affected by the existence of shearing stress. Stress of this 

 type, on the one hand, arises from the crystal growth itself, 

 and on the other hand is called into play by external forces. 

 The automatic adjustment of the internally created stress to 

 neutralize that provoked from without affords the key to all 

 structures of the nature of foliation. The mineralogical pecu- 

 liarities characteristic of the crystalline schists must find their 

 explanation in kindred considerations ; for it can be shown 

 that the chemistry of bodies under shearing stress differs in 

 important respects from the chemistry of unstressed bodies. 

 The result is seen in the appearance of a certain class of ' stress- 

 minerals ' where the dynamic element has figured largely in 

 metamorphism, while in the same circumstances the formation 

 of minerals of another class seems to have been inhibited. But, 

 while some of the general principles governing the effects can 

 be formulated, the explanation on these lines of the observed 

 associations of minerals is a task for the future. It may be 

 that many of the particular problems involved will in time be 

 brought within the scope of laboratory experiment. 



CONDITIONS GOVERNING METAMORPHISM. 



The conditions governing metamorphism are temperature 

 and shearing stress, with uniform pressure as a factor of less 

 general importance. If the orogenic forces are sufficient to 

 maintain shearing stress everywhere at its maximum, the stress 

 itself becomes a function of temperature, since this determines 

 the elastic limit, and the principal conditions of metamorphism 

 come to depend upon a single variable. This degree of sim- 

 plification, however, is not to be expected universally. One 

 disturbing factor is the local rise of temperature sometimes 

 caused by the mechanical generation of heat in the crushing of 

 rock-masses. 



ANOPHELINES. 



Referring to the note in The Naturalist for November last, 

 page 339, a further circular has now been issued by Mr. D. J. 

 Grove, c/o Medical Officer, Lccal Government Board, Whitehall, 

 S.W. I., in which he states that anopheline mosquitos may 

 be distinguished from culicines by the following characters : — 

 {a) The attitude : An anopheline mosquito resting on a wall, 

 holds itself so that its body (which forms almost a straight line 

 with the head and proboscis) projects so as to form a distinct 

 angle with the substratum. In the case of the culicine mos- 

 quito, the body (which is hump-backed in appearance) is held 

 parallel to the supporting surface, (b) The length of the palpi 



igi8 May 1. 



