640 MICROSCOPIC GEOLOGY. 



confirm, whilst it was itself confirmed by, the idea thought most 

 probable on other grounds by Mr. Darwin, that the Pampean 

 formation was slowly accumulated at the mouth of the former 

 estuary of the Plata, and in the sea adjoining it. 1 A similar line 

 of inquiry has been of late systematically pursued by Mr. R. C. 

 Sorby ; who has applied himself to the microscopic study of the 

 composition of fresh- water marls and limestones, by ascertaining 

 the characters and appearances of the minute particles into 

 which shells resolve themselves by decay, and by estimating the 

 relative proportions of the organic and inorganic ingredients of 

 a rock, by delineating on paper (by means of the camera lucida) 

 the outlines of the particles visible in thin sections, then cutting 

 them out, and weighing the figures of each kind. 2 



451. It is obvious that, under ordinary 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 information may be drawn ; and the 

 inspection of a microscopic fragment will often reveal, with the 

 utmost certainty, the entire nature of the organism of which it 

 formed part. In the examination of the minuter fossil Corals, 

 and of those Polyzoaries ( 325) which are commonly ranked 

 with them, the assistance of the microscope is indispensable. 

 Minute fragments of the "test" or "spines" of Echinodermata, 

 and of all such Molluscous shells as present distinct appear- 

 ances of structure (this being especially the case with the Brachio- 

 poda, and with the families of Lamellibranchiate bivalves most 

 nearly allied to them), may be unerringly identified by its means, 

 when the ex^rnal form of these fragments would give no assist- 

 ance whatever. In the study of the remarkable ancient group 

 of Trilobites, not only does a microscopic examination of the 



casts which have been pre- 



Fl Q- 343 - served of the surface of their 



eyes (Fig. 343), serve to show 

 the entire conformity in the 

 structure of these organs to 

 the "composite" type which 

 is so remarkable a character- 

 ^__ istic of the higher Articu- 



Eye of Troobtte. lata ( 383), 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. 3 It is in the case of 

 the Teeth, the Bones, and the Dermal skeleton of Vertebrated 

 animals, however, that the value of Microscopic inquiry becomes 

 most apparent; since the structure of these presents so many 

 characteristics that are subject to well-marked variations in their 

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



1 See Mr. C. Darwin's "Geological Observations on South America," p. 32. 



2 See " Quart. Jonrn. of Geolog. Science,' 1 1853, p. 344. 



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

 Society, p. 19. 



