ORIGIN OF ROCK-FORMATIONS. 639 



are known to exist at present ; but there are some which differ 

 from it so little, that a knowledge of their structure helps mate- 

 rially to elucidate points, which would otherwise be rendered 

 obscure in it through the changes induced by fossilization. 1 



450. The foregoing details, taken in addition to the facts of 

 like nature that have been mentioned in previous parts of this 

 work (as, for example, in 294), will serve as examples of the 

 essential importance of microscopic investigation, in determin- 

 ing, on the one hand, the real character of various stratified 

 deposits, and, on the other, in elucidating the nature of the 

 organic remains which these may include. The former of these 

 lines of inquiry has not yet attracted the attention which it 

 deserves; since, as is very natural, the greater number of Micro- 

 scopists 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 question of 

 extreme interest to the Geologist, to determine how far these 

 had their origin in the disintegration of organic structures; and 

 much light may often be thrown upon this question by careful 

 microscopic analysis. Thus the author having been requested 

 by Mr. Chas. Darwin, about twelve 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, not- 

 withstanding the difference in their mode of aggregation. For 

 the Chilian tufa is obviously composed in great partof 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 decomposition. 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 frag- 

 ments of shell was altogether wanting in the Pampas deposit, it 

 could not be doubted that the materials of both were the same, 

 those of the Pampean formation having been subject to greater 

 comminution than those of the Chilian ; and this view served to 



1 See the Author's Memoir on the Microscopic Structure of Nummulite, Orbitolite, and 

 Orbitoides. in the "Quart. Journ. of the Geolog. Society," for Feb., 1850; and the ad- 

 mirable " Description des Animaux Fossiles du Groupe Nummulitique de Tlnde," by 

 MM. D'Archiac and Jules Haime. 



