202 PROCEEDINGS OP THE 



at Sunbury and Hampton. The President said that there were 

 several species of Azolla, and that the plants sometimes lived 

 through a mild winter in this country. Its chief interest was 

 on account of the alga Anahaena azollae living in the chambered 

 leaves. The plant comes from South America. 



The Hon. Secretary then read two notes from Mr. E. M. Nelson. 

 In the first Mr. Nelson pointed out inaccuracies in the figure and 

 description of Conochilus volvox in the Micrographic Dictionary. 

 The points referred to were explained by means of figures on the 

 blackboard. In the second Mr. Nelson gave an interesting account 

 of " The Birth of a Flagellate." He was watching a small cluster 

 of four green pear-shaped bodies, enclosed in a gelatinous envelope. 

 It was ISjxm diameter, and had anchored itself to the cover by one 

 of its flagellae, each of the green pear-shaped bodies having one. 

 While watching it one flagellum got stouter than the other, which 

 gradually disappeared. Then the pear-shaped bodies became 

 paler, and eventually vanished. Bright granules grew in the 

 gelatinous parts, and finally the organism became a pear-shaped 

 flagellate with a proboscis-like flagellum. The time taken was 

 two hours. Votes of thanks were accorded to Messrs. Akehurst, 

 Burton and Nelson for their communications. 



The President then called upon Mr. C. H. Cafiyn to deliver his 

 lecture on " Rocks and their Microscopic Structure." Mr. 

 Cafiyn started by showing on the screen a photograph of his 

 machine for cutting rock sections, and gave a brief description of 

 the method adopted, illustrating his remarks by means of a series 

 of preparations showing difierent stages in the making of a section 

 from the rough hand specimen to the finished slide. Nicol, the 

 inventor of the polarising prism that bears his name, was appar- 

 ently the first to cut thin sections of minerals and silicified fossil 

 wood about 1828. The method described is practically the same 

 as that used now, except that chips were used instead of slice^. 

 The first real use of thin sections of rock for petrological examina- 

 tion was made by H. C. Sorby in 1850, when he ground sections of 

 calcareous grit, and described the microscopical structure of sand- 

 stone. Very little work on this line was done in England except 

 by Sorby, although it was taken up in Germany by Oschatz, and 

 real microscopical petrology can safely be said to start with the 

 publication of Zirkel's book on Rocks in 1863. 



Rocks are divided into three main groups — igneous, sedi- 



