OF MINERAL BEDS, OR STRATA. 19 
and occur in so great a number, as to be decisive in regard to the 
origin of the whole. If a rock made up chiefly of rounded peb- 
bles, be found to contain imbedded shells, and another rock agree 
with it most accurately in composition and structure, except that 
the shells are wanting, there is ground for the belief that similar 
causes have operated in the production of both. The distinction 
between the primitive and secondary rocks is said to have been 
first clearly marked and stated, by Lehmann, a German philoso- 
pher, about the year 1756. 
OF MINERAL BEDS, OR STRATA. 
11. If a rapid stream empty its waters into a lake, it will carry 
down and deposit upon the bottom of the lake, gravel, sand, and 
clay, so that in the lapse of ages the hollow will be filled up and 
the lake disappear. If we then dig into this mass of alluvial de- 
posits, we shall find it made up of layers of different texture and 
composition. Sand, gravel, and clay will succeed each other, 
with a great variety in both the order and the thickness of the 
layers. 
The appearances which would be presented by the bed of a 
lake, or an arm of the sea that has been filled up, are exhibited 
on a much larger scale, by the fragmented or secondary rocks ; 
but the different layers have a thickness that entitles them to the 
name of beds. Instead of the words, bed, and beds, Geologists 
employ the Latin, stratum and strata, having exactly the 
same signification, and state respecting certain rocks that they 
are stratified ; meaning thereby, that they are in beds placed one 
above another ; and this is true, as well of the primitive as of ' 
those that are secondary. The whole series of rocks is iherefore 
divided with reference to their structure into two other great 
classes ; the stratified and unstratified. 
When a considerable portion of the crust of the earth, composed 
of a number of strata, is taken into view at one time, it frequently 
happens, that it is seen to be made up of a few large masses, bear- 
ing little resemblance to each other, but the parts of which, 
though with slight shades of difference, are much alike, and of 
which it is therefore inferred that they were produced by the 
continued operation of the same causes. Such a body of resem- 
bling rocks is called a formation ; but much latitude obtains in 
the books in regard to the application of this term. The body of 
sandstone, lying east and south of the Universit) T , exhibits many 
varieties of composition and texture in its remote parts, but 
constitutes only a single formation.* 
* The French language, richer in the terms of art and science than our own, 
employs the words, terrai?i, formation, and sous-formation; the first being the 
name of a genus, of which the others, are species and sub-species. A terrain 
may include several formations. But complaint is made as of the English 
word formation, that there is a want of precision in their application and 
meaning. 
