192 



Oji Formations, 



[March^ 



at present known, do not exceed 50 or 60 ; and amongst these 

 are several enumerated in the list of simple earthy minerals. 

 Some mineralogists have considered them as more numerous ; 

 and have described every variety of composition as a distinct 

 species, and in place of 50 or 60 species have enumerated 

 several hundred. But the greater number of these are mere 

 varieties of the common mountain rocks^ of limited extent, often 

 not exceeding a few fathoms in length and breadth. This error 

 is owing to a misconception of what is understood by a mountain 

 rock. A mountain rock is distinguished by its position in regard 

 to other rocks, its magnitude, under which I include its length, 

 breadth, aud thickness, in the tract where it occurs, and the 

 proportion and kind of minerals of which it is composed. 

 Viewing them in this manner, it is not sufficient that they 

 exhibit varieties in structure or composition, they must also have 

 a determinate position and considerable magnitude, and the 

 certainty of the distinction is augmented if they possess univer- 

 sality of distribution. Most of the mountain rocks are univer- 

 sally distributed : thus the compound rock, known under the 

 name granite, and which is so abundant in Europe, occurs in 

 China and Van Diemen's Land, at the Cape of Good Hope and 

 in Bengal, in Brazil, Mexico, and Canada ; and in all these 

 countries it has the same characters. From this fact it follows 

 that we can with confidence draw inferences in regard to the 

 geognostic characters of rocks in one country from what has been 

 observed in another, and consider these as applicable, on a 

 general view, to the whole crust of the earth. Experience does 

 not contradict this inference; on the contrary, it is confirmed by 

 the investigations of geognosts in the most distant countries. 

 This universality of the distribution of formations, consequently 

 of the laws of the formation of the earth, has procured for 

 geognosy a place amongst the physical sciences ; and in it lies 

 that charm which leads so irrestibly to geognostic investigations, 

 as soon as we begin to occupy ourselves with the study of this 

 branch of knowledge. It would wear out the patience of the 

 most persevering inquirer, and would convey no very satisfac- 

 tory information, if a new set of rocks, or a new arrangement of 

 those already known, were to be studied in every small tract of 

 country. We might as well pretend to arrange and number the 

 grains of sand on the sea shore. In every country of consider- 

 ' able extent we find the outline of the structure of the whole 

 globe. 



Some series of rocks, however, do not possess this universality; 

 they appear in bason-shaped hollows, or in countries that have 

 been formerly in the state of lakes, or in level plains resting on 

 alluvial matters : their extent is, comparatively, inconsiderable ; 



