64 THE GRANITES OF MAINE. 
particles in the case of the fine-textured granites is estimated. Any 
peculiarities of texture, rift, etc., can also be noted. 
Proportions of minerals. — A method has been devised by Rosiwal, 
of the Austrian Geological Survey," by which the approximate pro- 
portions of the chief minerals (feldspar, quartz, mica, hornblende) 
and their average size can be determined. This consists in tracing 
a network of lines intersecting one another at right angles upon a 
polished granite surface, at intervals so far distant that no two 
parallel lines will traverse the same mineral particle. The total 
length of the lines is measured, then the diameters of all the particles 
of each kind of mineral are added separately and their proportion 
to the total length of the lines obtained. r J die average size of the 
particles of each mineral can be also calculated from the same 
measurements. Although this method was primarily designed for 
application to the coarse and medium granites, it can be extended 
also to the finer ones by drawing the lines upon camera-lucida draw- 
ings made from thin sections of such granites under polarized light. 
As the quartz is the source of the vitreousness of the rock the deter- 
mination of its amount is important. The incompleteness of the col- 
lection of polished specimens of Maine granites and the short limit 
of time available have alone prevented the application of this method 
in the preparation of this report, but the method Avas experimentally 
applied to a specimen of the coarse reddish granite from Hardwood 
Island, near Jonesport, and the results are given on page L73. 
Polls A.— Besides the manifest object of this test it also facilitates 
exact descriptions of color and comparisons between different granites. 
The size of the mica plates determines the brilliancy and durability 
of the polish more than does their number — that is, a considerable 
number of very minute mica plates is not objectionable. 
Hardness. — As pointed out by Hawes h the hardness of certain 
granites i> not due entirely to the quartz, which is always equally 
hard and brittle and which the tools do not cut but crush, but to the 
feldspar, which is of variable hardness and, it might be added, has 
different cleavages, and the proportion of which in relation to quartz 
also varies. Rosiwal.'' adopting a principle established by Toula, 
takes a piece of smooth unpolished granite of about 2 grams weight 
and rubs it with emery (of 0.2 mm. diameter of particle) upon a 
glass or metal plate for <» or 8 minutes until the emery loses its 
effectiveness. The granite is then weighed again ami its loss of 
"Sec Rosiwal, August, Ueber geometrische Gesteinsanalysen ; ein einfacher Weg zur 
Eiffermassigen Feststellung des quantitatsverhaltnisses der Mineralbestandtheile gemeng- 
ter Gesteine : Verhandl. der K.-K. geol. Reich sanstalt, vol. 32, pp. 143 17.",. 
"Hawes, G. W. (edited by Merrill), Granite: Building stones of the United States mid 
statistics of the quarry industry: Tenth Census, vol. 1i>, 1 sss, pp. n; is. 
c Neue Untersuchungsergebnisse iiber die HSrte von Mineralien und Gesteinen : Verhandl. 
K.-K. geol. Reichsanstalt, 1896, p. 488. 
