8 Sir R. I. Murchison’s Address on the 
natural base of the Silurian rocks. For English geologists 
should remember that this arrangement is not merely the issue of 
the view I have long maintained, but is also the matured opinion 
of these geologists in foreign countries and in our colonies, who 
have not only zealously elaborated the necessary details, but who 
have also had the opportunities of making the widest comparisons. 
n the continent of Europe, an interesting addition has been 
made to our acquaintance with the fauna of one of the older 
beds of the Lower Silurian rocks, or the Obolus green sand of 
St. Petersburg,* by our eminent associate, Ehrenberg. He has 
described and figuredt four genera and ten species of micro- 
scopic Pteropods, one of which he names Panderella Silurica ; 
the generic name being in honor of the distinguished Russian 
palzontologist, Pander, who collected them. It is well to re- 
mark, that as the very grains of this Lower Silurian green sand 
seem to be in great part made up of these minute organisms, so 
we recognize, in one of the oldest strata in which animal life has 
been detected, organisms of the same nature as, and not less 
abundant than, those which constitute the deep sea-bottoms of 
the existing Mediterranean and other seas. 
Calciferous sand-rock of North America. The Maclurea is in- 
deed known in the Silurian limestone of the south of Scotland ; 
but the Ophileta and other forms are not found until we reach 
the horizon of North America. Now, these fossils refer the zone 
of the Highland limestone and associated quartz-rocks to that 
portion of the lower Silurian which forms the natural base of 
the Trenton series of North America, or the lower part of the 
Llandeilo formation of Britain. The intermediate formation— 
the Lingula-flags or “zone primordiale” of Bohemia— having 
no representative in the north-western Highlands, there is neces- 
sarily a complete unconformity between the fossil-bearing crys- 
talline limestones and quartz-rocks with the Maclurea, Marchi: 
sonia, Ophiléta, Orthis, Orthoceratites, &c., and those Cambrian 
rocks on which they rest. 
_, 4 great revolution in the ideas of many an old geologist, in- 
iuding myself, has thus been effected. Strengthened and con- 
* See “ Russia and the Ural Mountains,” 
+ Monats-Bericht d. Kénig. Akad. der Wiss, Berlin, 18 April, 1861. 
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