
io 
" Parr IL. § iii] MACROSCOPIC CHARACTERS OF ROCKS. 93 
the foliation planes of schistose rocks and upon the lamine of 
- micaceous sandstones. As this silvery lustre is almost invariably due 
to the presence of mica, it is commonly called distinctively micaceous. 
A metallic lustre is met with sometimes in beds of anthracite ; more 
usually its occurrence among rocks indicates the presence of metallic 
oxides or sulphides. 
5. Feel and Smell.—These minor characters are occasionally ~ 
useful. By the feel of a mineral or rock is meant the sensation 
experienced when the fingers are passed across its surface. Thus the 
hydrous magnesian silicates have a marked soapy or greasy feel. 
Some hydrous mica-schists with margarodite or an allied mica, like- 
wise exhibit the same character. Some rocks adhere to the tongue, 
a quality indicative of their tendency to absorb water. 
Smel]l.—Many rocks when freshly broken emit distinctive odours. 
Those containing volatile hydrocarbons give sometimes an appreciable 
hituminous odour, as is the case with some of the dolerites, which in 
central Scotland have been intruded through coal-seams and carbon- 
aceous shales. Limestones have often a feted odour; rocks full of 
decomposing sulphides are apt to give a sulphurous odour; those 
which are highly siliceous yield, on being struck, an empyrewmatic 
odour. It is characteristic of argillaceous rocks to emit a strong 
earthy smell when breathed upon. 
6. Specifie Gravity— This is an important character among 
rocks as well as among minerals. It varies from 0°6 among the hydro- 
carbon compounds to 3°1 among the basalts. As already stated, 
the average specific gravity of the rocks of the earth’s crust may be 
taken to be about 2°5, or from that to 3:0. 
The student will find this character of considerable advantage 
in enabling him to discriminate between rocks. He may acquire 
some dexterity in estimating even with: the hand the probable 
specific gravity of substances; but he should begin by determining 
it with a balance. Jolly’s spring balance is a simple and service- 
able instrument for this purpose. It consists of an upright stem 
having a graduated strip of mirror let into it, in front of which 
hangs a long spiral wire, with rests at the bottom for weighing a 
substance in air and in water. For most purposes it is sufticiently 
accurate, and a determination can be made with it in the course of 
a few minutes.’ 
7. Magnetism is so strongly exhibited by some crystalline rocks 
as powerfully to affect the magnetic needle, and to vitiate observa- 
tions with this instrument. It is due to the presence of magnetic - 
iron, the existence of which may be shown by reducing a rock to 
powder in an agate mortar, washing carefully the triturated powder, 
and drying the heavy residue, from which grains of magnetite 
or of titaniferous magnetic iron may be extracted with a magnet. 
1 Jolly’s spring balance can be obtained through any optician or mineral dealer from 
Berberich, of Munich, for nine florins. In the United States it is manufactured by 
Geo. Wade and Co., at the Hoboken Institute, 
