a ° “- 
. ~ * 
6 GEOLOGICAL SURVEY 
to the conclusion that they had an igneous origin. Chalk is converted into crystalline marble, and 
bituminous coal into anthracite, when they come in contact with trap dykes. In Chesterfield Dis- 
trict a trap dyke passes through the new red sandstone, baking it at the points of contact, so that it 
can scarcely be distinguished from over-bumed bricks. | 
CHAPTER II. 
Stratified Rocks.—Stratification.—Joints.—Slaty Cleavage—Primary Stratified Rocks.—Forma- 
tions.—Systems.—Neries and Periods.— Gneiss —Hornblende Slate.— Lime rock.— Mica Slate.— 
Quartz rock.— Talcose Slate-—Chlorite Slate—Clay Slate—Relation of Igneous and Metamor- 
phic Rocks.— Volcanoes and Earthquakes. Dip.—Strike.— Outcrop.—Metalliferous Veins.— 
Faults. 
It has been already stated that the stratified rocks consist of tabular masses or layers, called beds 
or strata, of mineral substances piled up one upon the other. The analogy existing between these 
strata and sedimentary accumulations known to result from the deposit of matters held in suspen- 
sion in water, has given rise to the names aqueous or sedimentary, by which they are sometimes 
designated. 
The term bed is often applied to a subordinate pr irregular layer, interposed between the strata. 
And a seam is a thin layer disposed between strata or beds. 
Stratification, Joints, Slaty Cleavage.—The parallel planes which divide aqueous deposits into 
lamine or layers of various thickness, and which are the result of deposition from water, are 
called planes of stratification, or bedding planes. 
They afford useful indications to the Geologist of the circumstances under which rocks were 
deposited. The mud and silicious matter found on the banks of rivers, deposited by freshets, which 
divide readily into thin leaves, offer a good illustration of this structure. Besides these, there are 
other divisional planes, quite different from these. They are called joints, and are common to both 
stratified and unstratified rocks. They are generally fissures, extending frequently to a consider- 
able distance, both vertically and horizontally, dividing rocks into forms more or less regular. The 
parallelism which they often observe is very remarkable. . When they cross each other, which is 
frequently the case, they divide the rocks into rhomboidal masses, adding much to their value as 
building materials. 
These joints are thought to be the result of mechanical force, produced by contraction and other 
forces, during the consolidation of the rocks. On Horse Creek, at Vaucluse, fine examples of this 
structure may be seen, and at various places on Steven’s Creek. 
Slaty cleavage differs both from stratification and joints. It is most conspicuous in clay slates, 
which owe their value in the arts to this structure. The planes in which common roofing slate 
