634 MICROSCOPIC GEOLOGY. 



Foraminifera ; and the presence of animals of this group 

 may be recognized, by the assistance of this instrument, in sec- 

 tions of calcareous rocks of various dates, whose chief materials 

 seem to have been derived from Corals, Encrinite stems, or 

 Molluscous shells. Thus in the "Crag" formation (tertiary) of 

 the eastern coast of England, the greater portion of which is 

 perceived by the unassisted eye to be composed of fragments of 

 Shells, Corals (or rather Polyzoaria, 325), and Echinodermata, 

 the microscope enables us to discover Foraminifera, minute 

 fragments of shells and corals, and spicules of Sponges ; the 

 aggregate being such as is at present in process of formation on 

 many parts of our shores, and having been, therefore, in all 

 probability, a "littoral" formation, whilst the Chalk (with other 

 formations chiefly consisting of Foraminifera) was deposited at 

 the bottom of deeper waters. Many parts of the Oolitic (secon- 

 dary) formation have an almost identical character, save that the 

 forms of organic life give evidence of a different age; and in 

 those portions which exhibit the "roe-stone" arrangement from 

 which the rock derives its name (such as is beautifully displayed 

 in many specimens of Bath-stone and Portland-stone), it is found 

 by microscopic examination of transparent sections, that each 

 rounded concretion is composed of a series of concentric spheres 

 enclosing a central nucleus, which nucleus is often a Foramini- 

 ferous shell. In the Carboniferous (palaeozoic) limestone, again, 

 well-preserved specimens of Foraminifera present themselves; 

 and there are certain bands of limestone of this epoch in Rus- 

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

 frequently repeated through a vertical depth of two hundred 

 feet, which are almost entirely composed of Foraminiferous 

 shells belonging to a genus now extinct, the Fusulina. 



448. It is not only, however, in the condition of organisms of 

 microscopic size, that the Foraminifera have contributed iix an 

 important degree to the formation of the solid crust of the earth ; 

 for the Nummulitic limestone, 1 which forms a band, often 1800 

 miles in breadth, and frequently of enormous thickness, that 

 may be traced from the Atlantic shores of Europe and Africa, 

 through Western Asia to Northern India and China, and over 

 vast areas of North America likewise, is composed of an aggre- 

 gation of larger bodies belonging to the same type ; the " matrix," 

 or rock-substance, in which these are imbedded, being itself 

 usually made up (as microscopic examination of their sections 

 demonstrates) of the comminuted particles of similar organisms, 

 and of smaller Foraminifera ; although it is sometimes composed 

 (as in the British beds of London Clay which include Num- 

 mulites) of accumulations of clayey or other inorganic particles. 

 The structure of the Nummulite itself, as elucidated by micro- 

 scopic examination, presents some extremely remarkable modi- 

 fications of the ordinary Foraminiferous type. It is composed 



1 The Pyramids of Egypt are made of this material. 



