1884.] Boulder from the Cambridge Greensand. 67 



collection. On placing the slide from the Cambridge Greensand 

 erratic under the microscope, the difference from the Charnwood 

 rhyolites is at once perceptible. Spherulites abound: the smaller 

 aggregated in elongated clusters so as to give an irregular banded 

 structure to the rock, which, indeed, on inspection with the lens, 

 can just be seen on the smooth surface of the original fragment. 

 The majority of these spherulites are about ^i^ inch diameter, but 

 they not seldom attain about yi^inch, and sometimes even -gL 

 The structure is radial, but with some irregularity, so the 'black 

 crosses' are not well defined. One or two darker concentric bands 

 (more deeply stained with brown iron oxide) are often visible, and a 

 great number are compound in structure, a larger outer ring en- 

 closing two or three little spherules, like seeds in a husk. Beyond 

 the outer clearer ring is generally an irregular (more darkly 

 stained) zone of radiating crystallites, like the rays to a sun. The 

 remaining part of the ground mass is occupied by interlacing clear 

 acicular crystallites interspersed with an almost black residuum, 

 exhibiting some tendency to radial aggregation. Scattered about 

 this ground mass are small crystalline grains of rather clear quartz, 

 with a few of decomposed felspar, and rather numerous granules 

 and trichites of iron oxide — probably magnetite. In one place 

 there appears to have been a small cavity partly filled by ' dirty ' 

 chalcedonic quartz. 



Thus the microscopic structure of the rock differs very de- 

 cidedly from any specimen which I have examined from Charn- 

 wood. It differs also from the old rhyolitic rocks of the Wrekin 

 and of North Wales. Although it has a certain family likeness to 

 all of these, enough to embolden one to suggest that it may have 

 been derived from some volcanic mass, now lost to sight, which 

 was ejected in the latest Precambrian epoch, I cannot venture to 

 refer it to any locality known to me in Britain. I have however 

 no doubt that the pebble described by Mr Watts (Geol. Mag. Dec. 

 2, Vol. VIII. p. 95) is from the same locality. Through his kindness 

 I have again had the opportunity of examining this, and though 

 the structure shewn by it is more distinctly banded and less 

 definitely spherulitic than in Mr Fordham's specimen there are 

 so many minute points of agreement, that I feel certain both have 

 come from the same volcanic district. 



(2) On Critical or apparently neutral Equilibrium. By J. 

 Larmor, M.A. 



In the October Proceedings Mr Greenhill has translated the 

 solution of some equations which occur in the analysis appended 

 to my Note on Critical Equilibrium into Jacobi's elliptic function 

 notation. I may observe, however, that the general solution of 



5—2 



