362 
Directions for making Geological \ t$~c. 
[Dec. 
The endeavours of botanists to determine the impressions of plants found in the 
shale, in the coal measures, and in other strata, have been less successful, as they 
appear to be quite different from any now existing. They are considered to have 
been gigantic grasses or reeds, and some palms, and may be supposed to have 
required a greater degree of heat and moisture for their production than what 
exists in our present atmosphere. This has led some to imagine that the earth has 
been continually, but gradually, falling in temperature. This observation, and the 
opinions mentioned as held by the supporters of one of the theories of the for- 
mation of the earth, which supposed an internal or central heat, independent of 
that which is communicated by the sun, led to the investigation whether a differ- 
ence of temperature could he observed in descending into mines. The observations 
were made at different depths, in several mines, and the thermometers were let 
into the rock ; the conclusions from them rather indicate an increase of tempera- 
ture in descending. The thermometers, however, cannot well be supposed to have 
been kept entirely free from the influence of the miners’ light, blasting, &c. &c. ana 
also the greatest depth which has been reached below the level of the sea is only 
about 1200 feet, or nearly 1555 ® of the earth's semi-diameter. Mines in mountain 
districts, are, however, much deeper, some double that depth from the surface, or, 
if the greatest height and lowest level which has been examined be taken for the 
fraction, it gives about 157,0 part of the earth’s radius, which is therefore the P° r 
tion known, or rather partially examined by geologists. The experiments, however, 
of Dr. Maskelyneand Mr. Cavendish, (Philosophic Transactions,) give the mean 
specific gravity of the earth at nearly double that ot the rocks composing its crus , 
or equal to five times that of water ; and from the more recent investigations ma 
by Laplace, he considers the layers of different specific gravity to be concentnc 
the centre of gravity of the earth, their density increasing as they are situa _ 
nearer to the centre. The same philosopher has inferred, from the effect of the 
on the the earth’s surface, that the mean depth of the ocean is about 1 1 English nil • 
Mineralogy is the branch which is most requisite to the Geologist, since a kno' 
ledge of the simple minerals which enter into the composition ot rocks, is esseii • 
The manner in which some knowledge of this branch may be acquired, has a re‘ 
been mentioned. In the description of the rocks it has been seen, that the diffe 
species pass into each other: no such transition can take place between the specie 
mineralogy ; but to be capable of being determined, the minerals must be m a so ^ 
unaltered state, and in many species crystallized, or have faces of cleavage, an . 
a sufficient size to observe the characters; otherwise they can only be deter/n 
indirectly, by inference from a perfect specimen, or suite of specimens, estabiis > 3 
the identity or agreement of character, into the smaller and less distinct. . 
The minerals composing rocks arc frequently so minute as only to be made o 11 
this manner ; but when that is once done, the eye acquires the habit of judging 
similar cases, fylany of the greenstones, the base of porphyry, &c. are of thisna 
In these pastes, feldspar appears to be nearly always the predominant mgre 1 
Mechanical analysis is a mode which has been resorted to in such cases ; nan* 
by reducing the ore to almost a powder, and by a microscope or lens, observing 
appearance of the parts, as to the cleavage, &c. See. on these being agitated in a v 
sel of water, they will separate, according to their specific gravity. Monsr. Cor 1 
mentions having pounded lava, and also hornblende and feldspar, and 
the two latter, until under the microscope they gave the same appearance e , 
former, so as to be able to judge or estimate the relative quantities of each. 
inical analysis cannot well be applied in such cases, since from the different 111 
rals which compose the rocks, frequently changing in their relative quantities? 
result would not be satisfactoiy. 1 to 
In the system of Mineralogy by Mohs, no chemical properties are employ ett j r 
distinguish the minerals, since such cannot be observed without changing o 1 f , 
stroying them ; it is entirely founded on external character. The species so £ ‘ e t 
mined, however, and the chemical species, agree with very few exceptions : th a ^ 
the same component parts combining in tire same proportions give the same *° ; 
hardness and weight. This was assumed by Hauy in the definition of his spe cl 9l 
but lately it has been proved by Mr. Mitcherluh, of Berlin, that with some 
different substances would combine, giving exactly the same crystalline forhb.^ji 
hence the possibility of their combining with the base in variable proportions, ^ ;J ) g . 
accounts for some discrepancies in the analysis of Garnet and other miUy ^ 
Some chemical tests are useful, as the readiest mode of arriving at the knotf^li 
ot the mineral or rock, as for instance acid for lime. Muriatic acid, dilute^ , uy 
