ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxiil 



cies does not exceed 40, but the majority are new. They belong tc 

 the genera Equisetites, Sagenopteris, Cycadites, Zamites, Otozamites, 

 Araucarites, and Br achy phy Hum. The Cycadece, and especially the 

 Otozamites, predominate. M. de Zigno is about to publish a mono- 

 graph of his highly important discoveries, and it is to be hoped that 

 British geologists will render him due assistance, the more so as all 

 students of the English and Scottish oolites must feel greatly inter- 

 ested by this announcement. 



But, on the risk of taking subjects out of the order of time, I must 

 not omit to notice progress in the old and favourite direction of the 

 vegetation of the carboniferous epoch and the origin and working of 

 coal. The papers by Mr. Dawson and Mr. Poole on the phsenomena of 

 the coal-formation of Nova Scotia are contributions to this subject of 

 very high interest, and are accessible in the pages of our own Journal. 

 An excellent sketch, not without original matter, of the natural history 

 of coal and the " Fossil Flora of the Mountain Limestone formation of 

 the Eastern Borders," by Mr. George Tate, appended to Dr. John- 

 ston's delightful work on the Natural History and Antiquities of the 

 Eastern Borders, well deserves the notice of the geologist and student 

 of fossil plants. Circumstances of commercial interest have directed 

 the attention of many men of science during the past year to the in- 

 vestigation of the nature of coal, and attempts at a strict and unmis- 

 takeable definition of what coal is has, I fear, after carefully reading 

 all that has been said upon it, taken up in vain much of the time and 

 thought of both philosophers and lawyers. Coal has become a geo- 

 logical chameleon. Opinions on this vexed question must necessarily 

 vary according to the point of view, whether chemical, or geological, 

 or mineralogical, or microscopical, at which we regard it. By making 

 an a priori rule as to what coal should be, any man may arrive at a 

 strict specific character, and more than one view of the matter may 

 be right. 



Petrological Inquiries. 



The often-discussed subject of cleavage, about which so many geo- 

 logists are at variance, has been treated in a fresh and novel manner 

 by Mr. Sorby, who has communicated an essay of singular interest, 

 "On the Origin of Slaty Cleavage," to the Edinburgh New Philoso- 

 phical Journal for last year. This diligent observer has called the 

 microscope to the aid of the hammer and clinometer. By an exami- 

 nation of extremely thin sections of rocks under high powers (that 

 which he recommends as most generally useful for the purpose in 

 view is about 400 linear), he has been enabled to throw new light on 

 some of the greater geological problems ; among others that of the 

 cause of slaty cleavage. For the examination of slate rocks he re- 

 commends the use of a polarizing microscope. The physical struc- 

 ture and the optical properties of the component minerals may be 

 identified thus, even when in grains less than y^-L^th of an inch in 

 diameter. A comparison of sections of uncleaved with those of cleaved 

 rocks, having similar mineral comjiosition, shows that the minute par- 



