1090 THE MICKOSCOPE IN GEOLOGICAL INVESTIGATION 



the Tertiary limestone of which Paris is chiefly built consists almost 

 exclusively of the shells of Miliolida>, and is thus known as miliolite 

 (millet-seed) limestone. In the vast stratum of nummulitic lime- 

 stone which was formed in the earlier part of the Tertiary period 

 the microscope enables us to see that the matrix in which the large 

 entire nummulites are imbedded is itself composed of comminuted 

 fragments and young shells of the same, together with minuter 

 Foramihifera. Similar organisms, with fragments of crinoids. 

 mollusca, coral, &c., are abundantly present in the Jurassic lime- 

 stones in this country, in those of Secondary age generally in 

 Europe, as well as in the Carboniferous and other Palaeozoic lime- 

 stones ; in fact, wherever subsequent changes have not rendered the 

 structure of the original constituents indistinguishable. Thus in 

 the great plains of Russia there are certain bands of limestone of 

 this epoch, 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. Again, those parts of the Carboniferous 

 limestone of Ireland which have undergone least disturbance can be 

 plainly shown, by the examination of microscopic sections, to consist 

 of the remains of Foraminifera, Polyzoa, fragments of corals, tfcc. 

 And where, as not unfrequently happens, beds of this limestone are 

 separated by clay seams, these are found to be loaded with ' microzoa ' 

 of various kinds, particularly Foraminifera (of which the Saccamina 

 has come down to the present time), and the beautiful polyzoaries 

 known as ' lace-corals.' 



Mention has been already made of Professor Ehrenberg's very 

 remarkable discovery that a large proportion (to say the least) of the 

 green sands which p'resent themselves in various stratified deposits, 

 from the Silurian period to the Tertiary era, and in that called 

 the Upper Greensand, is composed of the casts of the interior of 

 minute shells of Foraminifera and Mollusca, the shells themselves 

 having entirely disappeared. The mineral material of these casts 

 has not merely filled the chambers and their communicating 

 passages, but has also penetrated, even to its minutest ramifications, 

 the canal-system of the intermediate skeleton. The precise parallel 

 to these deposits presents itself in certain spots of the existing sea- 

 bottom, such as the Agulhas bank, near the Cape of Good Hope, 

 where the dredge comes up laden with a green sand, which on 

 microscopic examination proves to consist almost entirely of 

 'internal casts' of existing Foraminifera. 1 



It is, however, in the case of the teeth, the bones, and the dermal 

 skeleton of vertebrate animals that the value of microscopic inquiry 

 becomes most apparent ; since their structure presents so many 

 characteristics which are subject to well-marked variations in their 

 several classes, orders, and families that a knowledge of these 

 characters frequently enables the microscopist to determine the 



1 See Challenger Reports ; Deep Sea Deposits (Murray and Kenard), p. 378, 

 &c. The same volume describes and figures the microscopic structure of remarkable 

 manganese concretions, dredged at great depths in the ocean, and often associated 

 with organisms. 



