Wellington Philosophical Society. 529 



it, as he had no pretension to any knowledge of geology. But when last 

 year Mr. McKay asked him to examine microscopically this deposit, he 

 was impressed by so marked a difference between the diatom growth on 

 the surface-grasses and water-weeds and the diatoms in the earth itself, 

 and suggested to Mr. McKay to have a further examination of it. 

 It might be well, perhaps, to remind the meeting that by " diatoms " are 

 meant a number of excessively minute organisms existing in fresh or 

 salt water, which naturalists nowadays unanimously agreed to consider as 

 plants. These plants, almost inconceivably minute as they are, consist 

 very largely of a hard purely-siliceous skeleton, with (doubtfully) some 

 mucous substance either enclosed by or surrounding the flinty mass. In 

 life diatoms might be either attached by stalks to weeds, or free and stalk- 

 less, and in many instances were endowed with a power of motion in 

 the water which nobody had yet been able to satisfactorily explain. 

 When dead the flinty skeleton alone remained, and this, falling to the 

 bottom of the water, went to build up the substance known as " diatoma- 

 ceous earth," of which the present was a specimen. Further, in most 

 instances it was possible to distinguish living and dead diatoms by the 

 presence in the former of " endochrome," internal colouring -matter, 

 probably part of the mucous portion of the plant. Now, the difference 

 which, in 1891, he had noted in the material submitted by Mr. McKay 

 was that, in the surface-growth of diatoms, mostly still alive, the vast 

 majority of the plants seen belonged to a common, widely-spread genus — 

 Melosira, with some very minute Navicular — while in the deposit then 

 taken from 1ft. in the earth there are also large numbers of an 

 entirely different diatom, which Mr. Grove (the leading English autho- 

 rity) referred to the genus Aclmanthes. This change led him to suggest 

 to Mr. McKay a further examination, and the result was that ten samples 

 were this year collected and submitted to the speaker, one for each foot 

 of depth (besides the surface-growth), the w T hole deposit being 9ft. thick 

 Of this he had made very careful examination, taking five specimen slides 

 from each sample, or fifty in all ; and the result not only confirmed his 

 diagnosis of 1891, but revealed further rather curious changes in the 

 genera seen. Whilst Melosira and Navictda formed all through the vast 

 majority of the diatoms, Aclmanthes exhibited rather curious oscilla- 

 tions, being strikingly plentiful at certain depths, and exceedingly rare, 

 if not absent, at others. Moreover, there were three other genera — 

 Surirella, Pinnularia, and Epithemia (all of which are by no means un- 

 common in many New Zealand deposits) — which occurred in large num- 

 bers at the depths of 8ft. and 9ft., but which then ceased to appear, being 

 entirely absent from the higher parts of the deposit. These oscillations 

 of genera seemed to the speaker to be worthy of remark, and perhaps of 

 still further inquiry. Mr. McKay was of opinion that a period of several 

 thousand years must have elapsed since the deposit was first commenced. 

 If this was correct, it would appear that some changes, climatic or 

 other, must have occurred at intervals, resulting in the prevalence or 

 the diminution of the various genera named. The speaker founded no 

 theory whatever on the facts observed, merely relating these as seeming 

 to be worthy of record and possibly of further examination. ' 



Specimens of the diatoms were -then examined under the 

 microscope by the members. 



After the reading of the papers, Mr. McKay exhibited 

 with the lantern a series of views of New Zealand scenery, 

 chiefly of the West Coast. They were intended principally 

 to show the geological character of the country. 



34 



