MOERAKI BOULDERS.

Auckland Star, Volume LXVIII, Issue 31, 6 February 1937, Page 1 (Supplement)

MOERAKI BOULDERS.

FORMING IN AUCKLAND HARBOUR. NO VOLCANIC ORIGIN. The common theory that the wellknown and much-discussed Moeraki boulders arc unique is quite erroneous; so also the local myths concerning; volcanic origin. And to fay that these concretions—for such they are; nothing more, nothing less—have puzzled geologists is quite contrary to fact. Indeed, the Moeraki boulders have been the subject for much study and inspection, and j their origin is a matter of common knowledge among geologists in this country to-day. I Whence cane they? T'sis question, has been asked, and rightly so, vei y! many times since the boulders were fn-it discovered by the Maoris. The Moeraki boulders belong to a class of rocks known as concretions, very widely distributed in sedimentary rocks. If one were to look at the seacliffs round Moeraki and Hampden it would be noticed Mint the '■boulders' have formed within the local sedimentary strata (sandstones), and are exposed oil the beach as a result of removal of the surrounding soft sandstones by the erosive action of the sea. The mode

of origin of a "boulder" is as follows: \ The sandstone was originally deposited: as a marine sand heiK.uh the sea in early Tertiary times—say 50 million j vears ago. After uplift in relatively late times (perhaps live million years ago) the -and has been compacted to nj rather incoherent sandstone. Wat.-rs percolating constantly through the pores have carried small amounts of lime ill the form of bicarbonate derived from enclosed fossil shells. Here and there precipitation of lime in the form of carbonate has commenced in the pores of the sandstone around some nucleus such as a pebble or shell. As time lias gone on. precipitation has continued outward fr the centre, giving a more or les< spherical body consisting of sand linnlv cemented by lime carbonate —that i-. /Moeraki boulder. Radial cracking, due to shrinkage, sometimes accompanies growth of the concretion, and lime carbon-..te as yellowish or white crystals in the cracks. The growing. crystals exert a powerful pressure.l causing the cracks t., open further, and thick radiating veins of crystalline lime carbonate result. i Concretions of this type are found j in many localities in New Zealand. 1 though not usually so large or numerous | Lα at Moeraki. They are found in the \bbotsford mudstone near Dunedin and at many points in North Auckland, especially round Kaipara Harbour. They are alscl forming at the present time around shells in the Auckland Harbour silts. Local myths o f volcanic origin are. of course, quite erroneous, so also the idea that the Moeraki boulders are unique. They are. however, unusually clear examples of what the geologi-t calls septarian concretions.

AGE DETERMINATION IN FISHES. SOME INTERESTING METHODS.

(By A. W. B. POWELL.)

Everyone knows of the annual growth rings in trees and how a cross cut section in many kin.ls will reveal the exact age. .lust as plants undergo vigorous growth in spring and retarded growth in winter, which is recorded by alternato layers nr growth rings in a cross section of (lie trunk, 50 docs this same principle apply tc. iishes, the only difference being that the rings are recorded not in across section of the body but in their curious oar bones, known as otoliths, and in their bodv scales.

The ear of .1 fish serves (wo purpose?. N"ot only is it the seat of the sense of hearing, but it is also concerned with the maintenance of equilibrium; indeed, it is possible that the latter function is the more important of the two. Comparing the fish's ear with that of man. several important differences are at once apparent. The human ear consists of three parts, the external, middle, and inner ear. In the lish the first two of these are entirely wanting, there being no other portion of trumpet, no ear drum, and no Eustachian tube connectin- the middle ear with the pharynx, wiiile the inner ear itself is of much simpler design. The chief characteristics of the ear in the bony lishes is the presence of these solid bony concretions called otoliths. Growth and Fo.rn. of Otoliths. These otoliths or ear stones h.ne enamelled surfaces, and are provided Thev exhibit some variation in shape, and" size in different lishes, and as the form is fairly constant ill any particular species, they are of some importance in classification. The otoliths grow by the deposition of lime in layers on the outer surface, and as the rate at which this is laid down varies at different seasons if one is cut into thin sections and examined under a lens or microscope the laves formed in successive years ■ire clearly visible as a series of alternately light and dark concentric rings, similar to the zones on 11 scale or tile ,iii"s on a tree trunk. Thus by n studv of the otoliths it is possible to ascertain the a"0 of any particular fish, 'and this method of age determination has proved invaluable to those invcslipitins the life histories of certain food Fishes? Many fish otoliths are objects of rare, beautv, of fantastic shape womlertuliy fluted'nud serrated along the edges, and variously ornamented on the surface. A line collection of otoliths from local Ucs formed by the late Mr Hank Webster and recently presented to the. Auckland Museum by -Mrs. Webster, of Owens Koad, Auckland, reveals some interesting facts. Apparently s M of the li~h concerned is 110 criterion of the sue of the otoliths, for in many species these ear stones occur in inverse ratio to the size of the lish. The large khijillsli, for ovimnle has a set of tinv otoliths not exceeding a quarter of an inch in length, whereas the snapper, which is a smaller lish. ha* a set the largest of which exceed three-quarters of an inch

The Art of Scale Reading. The other principle of ape determiimtion in fishes is the art of scale reading. lit is well known that the number of scales on a iis=li remains constant throughout life, each scale growing in proportion to the growth of the fc h. le, orally speaking, a fish, or one o its scales does not "row steadily, lmt there is",l periodic variation in growth: there ~,,. p'-riods when growth is accelerated, followed hy periods when growth takes place more slowly, or in some cases is stopped altogether for H time. 1-Jic-se. variations in rate of growth are istiiiguishablc in the structure of the scaTe regular zones arranged from the centre to the edge of the scale. In many species of iWi increased Ti-owth takes place during the warmer part of the year, followed l.y diminished "i-owtli .luring the colder part of the, year. These scale-markings may thus be 'interpreted as summer rings and winter rin-s. although they do not always correspond to those Fea.sons on the land. The essential point is that there is a. periodicity in the building of different distimmishalKC parts of the scale which can be correlated with definite intervals of time.

■In reviewing the causes which make for increased growth, it is obvious that the amount of food which a fish call obtain will have a direct effect on its .'l-owlh. Another important factor is temperature. Within the limits which may be considered as normal for the existence of the species, a rise in temperature will cause .in increase,! rate of growth to take place, and conversely, .with a fall in temperature a diminution in the rale of growth occurs. At very I low temperatures lish will reuse to feed and in consequence stop growing, I although plenty of food may lis present. Peculiarities of Growth. ! It has been explained by Mr. A. E. i Ilell'ord. Chief Inspector of Fisheries,

Wellington, that this effect of temperature upon feeding and growth is prolial.lv the main cause of the formation nt summer and winter zones in th" j scaled It u.K.) usually happens that ',','[he wan'ner"seasons of the year. The (operation of these two factors, temperature and food supply, does not always follow a regular sequence for any one year. A burst of good f ling, accompanied by a spurt in growth, may ,'„',.,„. J,, ,[".<■ midst of a season which is othcruNc the lean period of the year. ,„ ,his way we get anomalous markings, sometimes termed "false rings.' which might result in the age being read. actually only'three years; or the growth throughout 'the year may be - steady nml uniform that the scj-oiini zones arc „„, elearlv distinguishable. It is clear thiit off the coasts of the northern continents or in the fresh-water regions of North America, where there is a conI siderable range between the lowest temperatures of winter and the highest temperatures of summer, the growth "f ; lish will show the most marked seasonal variation, and scale readings of 1 such lish will be correspondingly ea-ier

than in the case of regions where there is less variation between winter and summer condition*. Ape rinjrs are found al-> in niiiny shellfish, iHirtk-ulniiy Hie ■"""»■"> cockles. Only last week ! picked tip on an \.i.kl".in.l l>each n iiiiiiililT oi bivalve shells which showed well mnrkod pr..«-tl. rinp-. "I.i.h -ye interpreted m< winter r.->t pcvi.ul-. The distance from the edjie "' ' -'■■•'I "' the first vuv A was oxiftly ! <"*• ~„„.(. hetwcMl the iiiwediii;: I j- "■conl"lelllW. nVi«I»-liy j Uirougli the (ii-qscnl suiinnci- »ca»ou.