NATURE. VOL. 2 18. J U N E 22. 1968

significance for interregional correlation. A complete 1,000 - report will be presented elsewhere. Localities mentioned

MINIMUM FLUCTUATION were described by Kleinpell? unless otherwise noted. Calcareous nannoplankton floras of the mid-Caenozoic

FREQUENCY RANGE rocks of western North America differ from those in low latitude regions in having fewer species and in the relative abundance of individual species. These differences, like those of the corresponding planktonic foraminiferal

I faunas" resulted from oceanographic conditions which ' included a southward-flowing ancestral California Current

that brought cool water south to off Baja California. A biogeographic province was thus maintained off Cali- fornia, and this was different from that in low latitudes. As a result, not all nannoplankton species found in low

1 latitude regions occur in western North America, but of I those that do, several provide bases for interregional

lo-' lo-" lo-' 10-"0-' correlation. ( ANIN)' Rocks a t the type locality of the upper zone of the

Fig. 3. Minimun~ fluctuation as a function of frequency. Zemorrian stage in Los Sauces Creek, Venture County, California, contain species which range throughout most

above 400 MHz will suffer less. The safest frequency the Oligocene and lower Miocene. some selection seems to be the 800 MHz range5. however, are known only from Oligocene rocks elsewheres$9.

P. R. A~~~~~ These species, including Cyclococcolithus margaritae, Institute for Exploratory Research, Discoaster saundersi and D. adamanteus support correla-

US Army Electronics Command, tion of the upper Zemorrian with the Globorotalia opima Fort Monmouth, opima and Globigerina ciperoemis ciperoelzsis Zones of New Jersey. Trinidad6. These zones are considered to be equivalent

to the Chattian and lower Aquitanian Stages in Europelo. Received May 20,1968. Saucesian strata a t the type locality (Los Sauces Creek)

Orhug T. Aa. Transactioru of Chalmers University of Technology, Gothen- contain few nannoplankton. hi^ stage, however, lies burg: ~ w e d k , No. 299 (1965). Ldarons, J., and Allen, R. S., J . Geophys. Res., 72,11, 2891 (1967). between rocks containing plankton that indicate firm

Briggs, B. H., and Parkin, I. A,, J . Atmos. Terr. Phys., 25, 339 (1963). correlations with tropical zones. The Saucesian is there- ' drn~y Research and Development Newsmagazine, 8, No. 8, 6 (1967). fore equivalent with the G2obigerina ciperoenais ciperoensis, j T S Patent 3,262,116. Turborotalia kugleri and lower Catapsydrax dissimilis

planktonic foraminiferal zones and the equivalent Spheno- lithus ciperoemis and the lower Triquetrorhabdulus carinatus nannofossil zonesg. The foraminiferal zones

M i d - C ~ ~ ~ O Z O ~ C Calcareous Nanno- correlate with the Aquit,anian and Burdigalian Stages of

plankton from Western North America Europelo. Rocks of the lower Relizian Stage a t Los Sauces Creek,

CALCAREOUS nannoplankton provide sound bases for and a t Graves Creek, San Luis Obispo County, and Reliz interregional correlation of Caenozoic marine strata, and Canyon, Monterey County, California, yielded a character- are potentially equally useful for inferring ocemiographic istic nannoplankton flora. The presence of Triquetrorhab- conditions a t various times in the past. Recent d e b a t e l - V u l m carinatus, together with Helicosphaera ampliaperta, concerning correlation of the mid-Caenozoic benthonic occurs only within the Catapsydrax dissimilis foramini- microfaunal stages of western North America with plank- feral zone in Trinidad9. Upper Relizian nannoplankton tonic foraminiferal biostratigraphic zones of low latitudes, occur in the Catapsydrax staznforthi and ~lobigerimtella and with the European stages has prompted this study of insueta Zones. These three planktonic foraminiferal the calcareous nannoplankton. The fossils were studied zones have been correlated with the Burdigalian and part by optical and electron microscopic techniques. This of the Helvetian Stagessvlo. communication records the stratigraphic distribution Luisian and Mohnian nannoplankton were studied in (Table 1) of the principal species and discusses their rocks from Newport Bay1 and by others from the Experi-

Table 1. STRATIGRAPHIC DISTRIBUTION OX CALCAREOUS NANNOPLANKTON IN ROCKS OF THE MID-CAENOZOIC MIOROFAUNAL STAQES OF WESTERN NORTH AMERICA Zemorrian Saucesian Relizian Luisian Mohnian

Apertapertu n. sp. - - .. Ponlosphaera sp. - -. -

Cyclococcolithus margaritae Roth and Hay Coccolithus firidanus Roth and Hay - - -

Discolithina cf. D. anisotrema (Kamptner) Bramlette and Wicoxon --- Cyelococwlithus ne,ogammation Bramlette and Wilcoxon - - - - - - - - - - - - - - - - - - - Discolithina vi&&%forata (Kamptner ex Dellandre) Loeblich and Tappan --.------------ - - - -

Discoaster adamanteus Bramlette and Wilcoxon - - - - - - - - .. - - - - - - -. - - .- Coccolithus eopdagtcus Bramlette and .Riedel - - - - - - - - - - - - - - - - - - - -

Discoaster deflandrei Bramlette and R~edel - - - - - - - - - - Triquetrorhabdulus carinatus Martini ---------- SphenoliWw belemnos Bramlette and Wilcoxon - - - - - - - - - - 8. morifomis (Bronniman and Stradner) Bramlette and Wilcoxon ----- S. heteromowhus Dellandre ----- Discoaster druggi Bramlette and Wilcoxon D. divarieatue Hay Hdicosphaeru ampliaperta Bramlette and Wileoxon - - - - -

Discoaster vari7bilis Martini and Bramlette - - - - - - - - - - ~ - - - - - - - - - ~ . - - - - Helicosphaera carte+ (Wallich) Kamptner - - - - - -~ - - - - - - - - - - - - - - - - - - .- - - - C M 8 c u s medusotdes Kamptner . - - - - - - .. - - - - - Cydococcolithus leptapmus (Murray and Blackman) Kamptner - - - - - - .- - - - - - - Lithoetromation perdurum Deflandre Discoaster e d i s Martini and Bramlette - . - - .. - . . - .. . - - -

D. challenger3 Bramlette and Riedel - - - - . - - - - - - -

D. kwleri Martini and Bramlette - - .. Catinaster ca2yculw Martini and Branllette .. - - - - - C. eoalitus Martini and Bramlette - - - - - - Discoaster brouwari Tan Sin Hok - - - - - - D. hahamatus Martini and Bramlette D. boUii Martini and Bramlette - - -

NATURE. VOL. 21 8. JUNE 22. 1968

mental Mohole off central Baja California. The floras in these rocks are more difficult to correlate than those of older stages apparently because of more pronounced temperature differences between North America and the lower latitndes. Several species (Discoaster kugleri, D . variabilis, D . h,arnatus and Catinaster coalitus) indicate correlation with the "Globorotalin fohsi" sequence of zones, and the Globorotalia rnayeri and "Glohorotalia cultrata" zones. The "G. fohsi" sequence of zones is absent from the type sections of the European stages, and thus was arbitrarily assigned to the Helvetian6 because this term was recommended for uselo. The ot,her two zones are equivalent to the TortonianIo.

The stratigraphic occurrence of calcareous nannoplank- ton in rocks of western North America permits correlation with tropical area plankton zones and the type sections of the European stages in spite of differing oceanographic conditions between the two regions during the mid- Caenozoic. These correlations indicate that the Cali- fornia Zemorrian Stage is equivalent to the European Chattian, the Saucesian to the Aquitanian, the Relizian to the Burdigalian and lower Helvetian, the Luisian to the Helvetian and the Mohnian %o the upper Helvetian and lower Tortonian.

JERE H. LIPPS Department of Geology, University of California, Davis, California, 95616. Received May 6, 1968. ' Lipps, J. H., Tulane Stud. Geo1.,2,109 (1964).

Parker, F. L., J. Paleontol., 38, 617 (1964). Martini, E., and Bramlette, M. N., J. Paleontol., 37,845 (1963). ' Barnes F. E. Banner F. T. Blow W H. and Clark? W. J. Fundamentals

of ~ i d - ~ i r t i u r y ~ i r a t i y r k ~ h i c d Cbrreiation (Cambridge' Gniv. Press, 1962).

' Bandy, 0 . L., &Iic~opaleonlology, 12, 79 (1966). Lipps, J. H., J. Paleontol., 41, 994 (1967). ' Kleinpell, R. M., Mzocene Stratigraphy of California (American Assoc.

Petrol. Geol., Tulsa, Okla, 1938). Hay, W. W., et al., Trans. Gulf Coast Assoc. Geol. Socs., 17, 428 (1967). IIramlette, 51. N., and Wilcoxon, J. A., Tulane Bud. Ceol., 5, 93 (1967).

'" Rntsch, R . F., Geol. .%li~nbotnu, 44, 96 (1965).

Lake Lappajarvi, Central Finland: a Possible Meteorite Impact Structure SVENSSON~ has drawn attention to a most interesting cryptic structure-one more in the growing list of such cryptoexplosion structures. I would, however, question how much weight should be attached to the recognition of shock lamellae in quartz grains as a criterion of meteor- ite impact involvement. I have previously endorsed Bucher's scepticism regarding numerous so-called astro- blemes2, and since that publication Snyder and Gerde- manna have put forward what seems to me to be an over- whelming case for the endogenous origin of the numerous cryptoexplosion structures east of the Appalachians. Currie and Shafiqullah4 have recovered potassic t,rachyt,c and alnoite from the Brent Crater, Ontario. Both these accounts confirm my conclusions, and the Brent discovery supports the idea that some sort of highly explosive alkalic vulcanism produced these structures. It is apparent that, shatter cones and coesitc, both found in association with cryptic structures of this group, are not restricted to meteoritic cryptoexplosion struct,ures. One rnay well quostion whet,her shock lamcllntiorl iu quartz has arly more validity: surely it has been reported frorn the Brent and Holleford craters, which are clearly of endogenous origin ? (A fortuitous association of alr~oite with an astro- bleme is extremely unlikely, for alnoites are very rare igneous rocks associated, in most cases, with localized. highly explosive diatrerne activity, involvirlg carbonatites, and such endogenous eruptivity produced t,he Pretoria Salt Pan which appears just like many of the cryptoexplo- sion craters attributed to meteorite? impact explosion.)

Geologists who, like myself, have worked extsr~sively on carbonatites5 must realize that the supposed rrstrictiot~ on the energy involvement involved in volcanic explosions (accepted without question by rnany auth0ritit.s because it is repeated so often in the literature) is ~ i t ~ h o u t fo~ulda- tion. The sort of phase change reaction that must IF involved in the production of the immense carhorlatite explosion breccia fillings of sorne carbonatitct vents. and of the associated cone sheets ---a reaction produci~~g a colossal explosion at a point focus a t depth-does not seem to bc limited in any way by the strength of the ovcr- lying thickness of crustal rocks. Slicll an explosior~, involving adiabatic expansion, could, physicists assure me, involve a virtually unlimited amount of energy. and so we are left with the possibility of two rr~echanisms, one intornal and one of extraterrestrial or igi~~, capahle of producing these cryptoexplosion structures. No one would argue that there is much conflicting evidence: and this conflict applies to the Rieskessel, New Quebec Crater, Manacouagan Lake, Clearwater Lakes. Rosumtwi Crater. and even to Wolf Creek Crater. One thing, hon-ever. is clcar from the literature: there are a t present no valid criteria t,hat afford strong support for either thoor>-, except the actual associat,iorl of meteorit,ic rnatedal; and even where that is present, as a t Wolf Creek6*:. ono can reasonably h a r b o ~ ~ r jrist an iot,a of doubt'.

;I. ?+i.IrC'~cr. Department of Geology. University of West,ern Australia, Nedlands. Iteceived April 26, 1968.

Svensson, N-B., Xett~ru, 217, 438 (1968). ' McCall, G. J. H., iTatlcre, 201, 251 (1964).

Snyder, F. (;., and Gerden~ann, P. E., Arner. J. Sri . , 263, 465 (196.5). ' Currie, K. L., and Shaffqullah, M., Suture, 215, i.75 (1967). j iMcCi~I1, (3. J. H., Report L V ~ . 45 (Geol. Snrvey, Keuyu. 1958). 6McCall, O . J. H., Ann. N Y Aead. Sci., Art. 1'73 (2), 9i0 ' McCall, (;. .T. H., J . Ceol. Soc. Anstral.,14. 169 (196i).

Contrasting Origins of the Eastern and Western Islands of the Canarian Archipelago MOST geologists working on the Chnary Islands (Bigs. I and 2) believed that t,he indix-idual islands were conneetecl to one another and to the African continent during the Tertiary Biologists seern to have agreed unani- mously with this Recent evidence, however, shows that the hypothesis of a continuous ('anaria11 landmass during the Tertiary should he al,andoneci. Geological evidence from the eastern island of Fuertr- ventura indicates that this and the island of Larlzarote are underla.in by continental crust,. Gral~ Canaria, on the other hand, and possibly the other westcrn islands its well, seem t,o be oceanic in origin.

Most of the island of Fuerteventurn is made up of volcanic rocks, chiefly alkali basalts and related tufYs. These are deeply dissected late Tertiary shield volcarloc~~ and Quaternary preserved cinder cones and lava flows. In the western part of the island, this t)asaltic sequencatb overlies a basement with erosional unconforn~ity. Thin complex basement is made I I ~ of plutonic rocks (strati- form ultramafic rocks and a yorrngor circular syenitt~ complex1". It is cut by abundant, K.X.H. trending. steeply dipping dykes us~~al l y consisting of meta- rnorphosed basalts. These dykes, interpreted earlier as "spilitic" lava flows" also cut a complex of sedimentar?. rocks which are contact-metamorphosed but are not offset dong the dykes. Tho sequence of sedimentary rocks forms part of the basement to the west of the plutonic rocks, but the contact between the plutonic and tho sedimentary complex is not oxposed; several lines of evidence indicate the plutonic rocks to be younger thi111 the sediments1'.