634
THEORY AND PRACTICE OF ENGINEERING.
Book II.
has a granular texture, the quartz grains are united by a crystalline cement of the samemineral.
Gneiss is composed of quartz, felspar, mica, and hornblende, the same ingredients whichare found in granite, although the proportions vary. The character in which gneiss mostdiffers is in its having the mica and hornblende arranged in planes parallel to the stratifica-tion, so that it may often be cleaved into plates. Its stratification is, however, irregularand contorted ; it usually rests upon granite. The whole of the rocks of the gneiss system,after being deposited by water, have been acted upon by heat, and acquired a highly crys-talline character; they are wholly devoid of organic remains, and in Britain contain no dis-tinct fragments of other rocks, either angular or rounded.
Plutonic or Unstratified Rocks. — Granite is a compound crystalline rock , containingquartz, felspar, and mica ; each of these bodies are composed of several elementary sub-stances ; they are intimately joined together, but without any base or cement: they varyin quantity : felspar usually predominates, and mica is less frequently present; they differalso in magnitude, alternating from large to small grains. In some varieties the con-cretions of felspar and quartz are several inches in size, and the mica occurs in platesupwards of a foot square, whilst in others the grain is so small that the granite appearsnearly compact. The crystals of granite are seldom arranged regularly, as in gneiss, butare united in a confused crystallisation.
The Graphic Granite is a variety compounded of felspar and quartz, arranged so as toproduce a laminar structure. The felspar crystals seem first to have been formed, andafterwards the darker-coloured quartz.
Aberdeen Granite sometimes has the mica replaced by hornblende, and there are varietiescomposed of hornblende and felspar.
Porphyritic Granite contains large crystals of felspar, held together in a granitic base inwhich are specks of mica of an hexagonal form. The uniform character of granite seemsto indicate that, after its elements were mixed together, they were crystallised at the sametime and under the same process. The minerals which constitute the granitic as well asthe volcanic rocks are silica, alumina, magnesia, lime, soda, potash, and iron, and thepresence of these seven elements in certain proportions is more favourable to the graniticstructure: the fine grain it sometimes assumes is perhaps owing to the manner in which ithas cooled.
Granite composed of quartz two parts, felspar two parts, and mica one part, is repre-sented in the first coliran. Porphyritic granite in column the second, and composed of twoparts quartz, three parts felspar, and one part mica. In the third column is shown a binarygranite of three parts felspar and two parts quartz.
No. 1.
No. ‘2.
No. 3.
Silica
.
- 74*84
-
73 04
-
75-1
Alumina
.
- 1280
-
13 83
-
10-9
Potash
-
7-48
-
8-51
-
9-8
Magnesia
-
099
-
0-83
Lime
-
0-37
-
0-44
-
0-5
Oxide of iron -
-
1-93
-
1-73
-
0-4
Oxide of manganese
-
012
-
010
Fluoric acid
-
0-21
-
0-18
Granite, when reduced to very fine grains, cannot be distinguished from felsparporphyry.
Syenite is a compound of compact or crystallised felspar, united with hornblende andquartz: where the proportions are equal the first column shows the ingredients; whencomposed of equal proportions of quartz, felspar, and mica, they are found in the secondcolumn; and when of schorl rock and quartz in equal parts in the third column.
No. 1.
No. 2.
No. 3.
Silica -
- 69-91
-
63-96
-
6801
Alumina
- 10-37
14-32
17-91
Potash
- 4 ‘55
5-94
0*35 soda.
Lime -
4-86
3-73
014
Magnesia
6-26
5-94
2-22
Oxide of iron
2-69
4 06
6-85
Oxide of manganese
0 07
0-21
0-81
Fluoric acid
0-50
0-65
1-79
Greenstone is composed also of compact and crystallised felspar, hornblende, or augite ;its texture is sometimes earthy, but when crystalline it resembles syenite, the differencebeing only its green colour; its compounds are found in the first column.
Hypersthene rock is of a white or red colour, and the felspar is compact or crystallised ;its compounds are in the second column.