THE MICROSCOPE IN GEOLOGICAL INVESTIGATION. 313 



to the belief which Geological evidence has been gradually tending to 

 establish, that the sedimentary rocks which form the existing land, were 

 deposited in the immediate neighborhood of pre-existing land, whose de- 

 gradation furnished their materials; and consequently that the original 

 disposition of the great Continental and Oceanic areas was not very dis- 

 ferent from what it now is. 1 Further, the microscopic examination of 

 these Oceanic sediments reveals the presence of extremely minute parti- 

 cles, which seem to correspond in composition to meteorites, and which 

 there is strong reason for regarding as ' cosmic dust ' pervading the inter- 

 planetary spaces. Thus the application of the Microscope to the study 

 of these deposits, brings us in contact with the greatest questions not only 

 of Terrestrial but also of Cosmical Physics; and furnishes evidence of the 

 highest value for their solution. 



707. The application of the Microscope to the determination of the 

 materials of the sediments now in process of deposition on the Ocean- 

 bottom, leads us to another great department of Microscopic inquiry now 

 being extensively prosecuted, namely, Microscopic Petrology, or the 

 study of the Mineral materials and Physical structure of Rocks. For 

 although the Geologist has no difficulty in determining by his unaided eye, 

 with the use of simple chemical tests, the mineral composition of rocks 

 of coarse texture, and in distinguishing the fragments of previously exist- 

 ing rocks of which they have been built-up, the case is different with 

 those of extremely fine grain, still more with such as present an appa- 

 rently homogeneous, compact, and glassy character. For it is only by 

 the microscopic study of these, that any trustworthy conclusions can be 

 arrived at in regard to the mode in which they have originated, and the 

 changes they have subsequently undergone; and such study often reveals 

 facts of the most unexpected kind and the most striking significance. 

 Thus, many compact sedimentary rocks, whose homogeneous appearance 

 to the eye or the hand-magnifier 'gives no clue to their origin, are found, 

 when thin sections of them are examined microscopically, to be aggrega- 

 tions of minute rounded and water- worn grains (often less than l-1000th of 

 an inch in diameter) of Quartz, Felspar, Mica, soft and hard Clays, Clay- 

 slate, Oxide of Iron, Iron-pyrites, Carbonate of Lime, fragments of fossil 

 Organisms, etc., arranged without any trace of decided structure or crys- 

 tallization. In rocks exhibiting slaty cleavage, again, the direction in 

 which the pressure has been applied is indicated in a microscopic section 

 by the elongation or flattening out of some of the particles, with a sliding 

 movement of others. In regard to eruptive or igneous rocks, on the 

 other hand, the results of microscopic examination enable it to be stated 

 that whether possessing the hardest and most compact substance, and 

 presenting the most homogeneous and even glassy aspect, or existing 

 under the form of the softest and finest powder (like the dust-ash of vol- 

 canoes), the rocks of this class are characterized as a rule by the 

 minutely-crystalline character of their mineral conponents; and this even 

 when their vitrification seems to the eye so complete, as to forbid the 

 expectation of any such recognition. And in this manner a clue is ob- 

 tained to the sources of these rocks; which (there is now strong reason 

 to believe) have been formrd for the most part, if not universally, by the 

 melting-down of the rocks pre-existing in the neighborhood, and not 

 ejected (as according to the older theory) from the general molten inte- 



! See Prof. Geikie's Lecture on 'Geographical Evolution,' in the "Proceedings 

 of the Royal Geograpical Society," July, 1879. 



