700 ORGANIC CONSTITUENTS OF ROCKS. 



each, rounded concretion is composed of a series of concentric 

 spheres enclosing a central nucleus, which nucleus is often a 

 Foraminiferal shell. In the Carboniferous (palaeozoic) Limestone, 

 again, well-preserved specimens of Foraminifera present them- 

 selves ; and there are certain bands of limestone of this epoch in 

 Russia, varying in thickness from fifteen inches to five feet, and 

 frequently repeated through a vertical depth of two hundred feet, 

 over very wide areas, which are almost entirely composed of the 

 extinct genus Fusulina (§ 388) : thus prefiguring, as it were, the 

 vast deposit of Nummulitic limestone (§ 392) which marks the 

 commencement of the Tertiary epoch. — Mention has already been 

 made (§ 390, note) of Prof. Ehrenberg's very remarkable discovery 

 that a large proportion (to say the least) of the green sands which 

 present themselves in various stratified deposits, from the Silurian 

 epoch to the Tertiary period, and which in certain localities con- 

 stitute what is known as the Greensand formation (beneath the 

 Chalk), is composed of the casts of the interior of minute shells of 

 Foraminifera and Mollusca, the shells themselves having entirely 

 disappeared. The material of these casts, which is chiefly Silex 

 coloured by Silicate of Iron, has not merely filled the Chambers 

 and their communicating passages (Fig. 253, a, b), but has also 

 penetrated, even to its minutest ramifications, the Canal-system of 

 the intermediate skeleton (Figs. 256, 260). — Even this discovery 

 pales in interest before that more recent one to which it has led, 

 and which may be regarded as the most remarkable achievement of 

 Microscopic inquiry as applied to Geology; namely, the determina- 

 tion of the Organic nature of those Serpentine-Limestones in the 

 Laurentian Formations of Canada and elsewhere, which are pro- 

 ducts of the growth of the gigantic Foraminiferal Eozoon over 

 immense areas of the ancient Sea-bottom (§§ 396-400). This 

 discovery is alike interesting to the Physiologist and Zoologist, on 

 the one hand, and to the Geologist on the other. For it presents 

 to the former the Rhizopod type of Animal life, than which 

 nothing simpler can well be conceived (§ 325), in an aspect of 

 most unexpected magnitude : whilst to the latter it affords evidence 

 not merely of the prevalence of Animal life, but of its important 

 share in the production of Rock-formations, in Strata so far below 

 those in which Organic Remains had previously been detected, that, 

 to use the words of Sir William Logan, the appearance of the so 

 called 'Primordial Fauna' is a comparatively modern event. 



592. The foregoing General Summary, taken in connection with 

 the more detailed statements that have been made in previous parts 

 of this work, will sutflce to indicate the essential importance of 

 Microscopic investigation, in determining, on the one hand, the 

 real character of various Stratified deposits, and on the other, the 

 nature of the Organic remains which these may include. The 

 former of these lines of inquiry has not yet attracted the attention 

 which it deserves ; since, as is very natural, the greater number 



