177. The figure on the preceding page presents an ideal section of the Earth's Crust, exhibiting the order of succession in the stratified rocks, and the relative positions of the unstratified.

B, signifies the classification adopted by Dr. Buckland; and L, that of Mr. Lyell.

1. Quaternary or modern deposits.

b. 1. Granite and Quartz Veins

in Granite.

2. The deposits of the Terti- b. 2. Granite and Quartz Veins

ary.

3. The Chalk formation.

4. The Wealden.

5. The Oolite.

6. The Lias.

7. The Triassic system.

8. The Carboniferous system.

9. The Old Red Sandstone.

10. The Silurian.

11. The Cambrian.

12. Mica Slate.

13. Gneiss.

14. Granitic Rocks.

15. Volcanic Rocks.

a. 1. Porphyritic dike overly

ing the gneiss.

intersecting the porphyry,

gneiss, and mica slate.

b. 3. Granite Vein passing into

the transition rocks.

c. Greenstone dike of the transition period.

c. 1. Trap Dike intersecting a

dike of porphyry.

c. 2. Trap Dike of the Oolitic period.

c. 3. Lava Dike of the volcanoes of the Tertiary period.

d. Lava of active volcanoes.

e. Cave in the magnesian lime

stone.

f. Metaliferous veins.

a. 2. Dike of Porphyry passing g. An extinct volcano.

through the coal.

h. An Artesian well.

CHAPTER V.

THE STRATIFIED PRIMARY ROCKS.

178. IN describing the stratified rocks, we may begin with the most recent, in which the circumstances of deposition were very similar to those existing at the present time, and proceed to the most ancient strata whose origin was in a condition of nature widely different from the present, as regards both their mineral structure and fossil contents. But the chronological order appears to be more natural and satisfactory; commencing with the oldest strata, which are very widely extended, and more uniform in character, following the succession exhibited in nature, the facts may be delineated in the order of their occurrence. A knowledge of the phenomena of the older rocks is indispensable to a due appreciation of the characters of the more recent, since the latter are principally derived from the former.

179. The primary stratified rocks-the oldest sedimentary deposits are composed of the same minerals that constitute the plutonic, granitic rocks, upon which they lie, and by the abrasion of which they were produced. As a class they are silicious in mineral character, and crystalline in structure; exhibiting, however, a stratified arrangement. Their particles appear to have undergone less attrition, and subjection to a higher degree of heat than the more re

cent strata. No organic remains have been found in them;
vegetables and animals either did not live upon the earth
at the time of their deposition, or all traces of them
have been obliterated by the heat to which the rocks
have been subjected. Their thickness is variable, but th
usually very great; in some instances exceeding twenty
miles. They are more widely extended than any other class
of strata, and where they form the surface they present bold,
rugged, and mountainous outlines. Their order of super-
position is not invariable; gneiss, however, usually lies
directly upon the granite, and mica-slate succeeds. This
class of rocks has been denominated metamorphic (§ 109.)
Veins and dikes of the igneous rocks, granite, porphyry,
and greenstone, with metaliferous veins are frequently
found traversing them.

Fig. 88.

Gneiss.

180. The term gneiss-derived from a Saxon word signifying to sparkle-is applied to a crystalline aggregation of quartz, feldspar, and mica, distinctly stratified, in which the feldspar is less abundant than in granite, and the quartz is in fine grains. The mica gives to the rock a glistening aspect. Gneiss passes insensibly into granite, so that it is often difficult to say where the granite terminates and the

A

gneiss begins. The lamina and strata are usually parallel to each other, and to the surface of the igneous rock upon which they lie, but frequently exhibit remarkable contortions, indicating a previous state of great flexibility. Dr. M'Culloch observes: "Imagination can scarcely conceive an intricacy of flexure of which a resemblance could not be found in the gneiss of the Western Isles of Scotland."

Distinct crystals of feldspar are sometimes found in gneiss; it is then termed porphyritic. When talc takes the place of mica, the rock is called protogine. If hornblende is added to the ordinary constituents of gneiss, the rock is termed syenitic. A section of graphic granite parallel to its imperfect laminæ, fig. 89, closely resembles a transverse section of gneiss.

181. Mica-slate differs from gneiss, in the absence of feldspar, the greater predominance of mica, and the more perfect slaty structure; it often, how

ever, passes gradually into gneiss. Perfect crystals of garnet and staurotide sometimes occur in this rock in as large proportions as either of its constituents, it is then denominated garnetiferous and staurotidiferous. Mica-slate passes by insensible gradations into clay-slate, and into talcose-slate.

182. Hornblende-slate is composed principally of hornblende, with a variable quantity of feldspar, quartz, and mica. When the hornblende and feldspar occur mixed in equal quantities the characters of the rock correspond with those of greenstone. Hornblende-slate occurs in all parts of the primary series, but is most frequently associated with mica-slate and clay-slate, into which it passes by imperceptible gradations.

183. The essential ingredient of talcose-slate is talc, which is sometimes pure, but more frequently mixed with feldspar, quartz, and mica. Steatite or soap-stone is a variety of this rock in which tale greatly predominates, is of a grayish green color, greasy feel, and is easily cut. Chlorite-slate is another variety, of a compact texture, and green color. Talcose-slate is a metaliferous rock, and frequently the repository of gold.

184. Serpentine, as stated in § 164, occurs both as a stratified and unstratified rock; in the former case exhibiting the characters of strata. As a metamorphic rock it is associated principally with talcose and hornblende slates.

185. Primary limestone, is a crystalline limestone, frequently pure white, and close grained, constituting the marble of sculpture; but is sometimes compact, and darkcolored. It occurs in thick beds, and in thin leaves. It alternates with gneiss, mica-slate, and clay-slate, and then usually contains some crystals of mica, quartz, and feldspar. It has been found in syenite, hornblende, and granite, entirely destitute of any trace of stratification, and even sometimes in the form of veins.

186. Quartz rock occurs interstratified with each one of the primary rocks. When pure it is granular, crystalline, with little or no tendency to divide in parallel beds; but when mixed with feldspar, mica, hornblende, talc, or clay slate, it exhibits regular stratification. In some instances it is eminently arenaceous-a mass of sand, exhibiting mechanical structure, and apparently made to cohere by the heat to which it has been subjected.

187. Clay-slate, or argillaceous schist is a rock common to the metamorphic and fossiliferous series. It consists of a very fine mixture of mica or talc with quartz, a large pro