310 THE MICROSCOPE AND ITS REVELATIONS. 



deposits presents itself in certain spots of the existing sea-bottom, such 

 as the Agulhas bank near the Cape of Good N 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 Foramini- 

 fera, that must have been formed by the chemical replacement of their 

 protoplasmic bodies by ferruginous silicates precipitated from the Sea- 

 water. And this fact gives the clue to the interpretation of the condi- 

 tions under which the 'Eozoie Limestone' of Canada ( 497), formed on 

 the sea-bottom of the Laurentian epoch by the extension of continuous 

 Foraminiferal growth resembling a Coral reef, became interpenetrated 

 with a like deposit of green silicate of magnesia (serpentine), of whose 

 presence in large amount in the sea-water of that period there is ample 

 evidence. The determination of the organic nature of this Serpentine- 

 limestone, which is one of the lowest members of a series of 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 9 is a comparatively modern event, may be regarded as 



the most remarkable achievement of 



FIG. 491. Microscopic inquiry as applied to 



Geology. 



703. It is obvious that, under or- 

 dinary circumstances, only the hard 

 parts of the bodies of Animals that 

 have been entombed in the depths of 

 the earth are likely to be preserved ; 

 'but from these a vast amount of in- 

 Eye of Triiobite. formation may be drawn ; and the in- 



spection of a microscopic fragment 



will of ten reveal, with the utmost certainty, theentire nature of the organism 

 of which it formed part. Minute fragments of the tests or spines of all Echi- 

 nodermata, and of all such Molluscous shells as present distinct appearances 

 of structure (this being especially the case with the Brachiopods, and 

 with Certain families of Lamellibranchiate bivalves), may be unerringly 

 identified by its means, when the external forms of these fragments 

 would give no assistance whatever. In the study of the important 

 ancient group of Trilobites, not only does a Microscopic examination of 

 the ' casts ' which have been preserved of the surface of their Eyes (Fig. 

 491) serve to show the entire conformity in the structure of these organs 

 to the ' composite ' type which is so remarkable a characteristic. of the 

 higher Articulata ( 626), but it also brings to light certain peculiarities 

 which help to determine the division of the great Crustacean series with 

 which this group has most alliance. 1 



704. It is, however, in the case of the Teeth, the Bones, and the Der- 

 mal skeleton of Vertebrated 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 fre- 

 quently enables the Microscopist to determine the nature of even the 

 most fragmentary specimens, with a positiveness which must appear 

 altogether misplaced to such as have not studied the evidence. It was 

 in regard to teeth, that the possibility such determinations was first made 



1 See Prof. Burmeister "On the Organization of the Trilobites," published by 

 the Ray Society, p. 19. 



