ORGANIC CONSTITUENTS OF ROCKS. 761 



of Microseopists are more attracted by those definite forms which 

 they can distinctly recognize, than by the amorphous sediments 

 which present no definite structural characters. Yet it is a mat- 

 ter of extreme interest to the Geologist, to determine how far these 

 may have had their origin in the disintegration of Organic struc- 

 tures ; and much light may often be thrown upon this question by 

 careful Microscopic analysis.— Thus the Author having been re- 

 quested by Mr. Chas. Darwin, more than twenty years since, to 

 examine into the composition of the extensive Calcareous deposit 

 which covers the surface of the Pampas region of South America, 

 and to compare it with that of the Calcareous Tufa still in process 

 of formation along the coast of Chili, was able to state that their 

 constituents were in all probability essentially the same, notwith- 

 standing the difference in their mode of aggregation. For the 

 Chilian Tufa is obviously composed in great part of fragments of 

 Shells, distinguishable by the naked eye ; the dense matrix in 

 which these are imbedded is chiefly made-up of minuter fragments, 

 only distinguishable as such by the Microscope ; while through the 

 midst of these is diffused an aggregation of amorphous particles, 

 that present every appearance of having originated in the yet 

 finer reduction of the same Shells, either by attrition or by decom- 

 position. In the Pampas deposit, on the other hand, the principal 

 part was found to be composed of amorphous particles, so similar 

 in aspect to those of the Chilian rock that their identity could 

 scarcely be doubted ; and scattered at intervals through these were 

 particles of Shell distinctly recognizable by the Microscope, though 

 invisible to the naked eye. Thus, although the evidence afforded 

 by the larger fragments of shell was altogether wanting in the 

 P; mpas deposit, it could not be doubted that the materials of both 

 were the same, those of the Pampean formation having been sub- 

 jected to greater comminution than those of the Chilian; and this 

 view served to confirm, whilst it was itself confirmed by, the idea 

 entertained as most probable on other grounds by Mr. Darwin, that 

 the Pampean formation had slowly accumulated at the mouth of 

 the former estuary of the Plata, and in the sea adjoining it.* — 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 Freshwater Marls and Limestones, by ascer- 

 taining 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.+ 



* See Mr. C. Darwin's " Geological Observations on South America," 

 p. 32. 



t See his successive Memoirs in " Quart. Journ. of Geolog. Science," 

 1853, p. 344, and subsequently. 



