Silurian strata occasionally horizontal.-The Silurian strata throughout a large part of the province of Skaraborg, in the south of Sweden, are perfectly horizontal; the different subordinate formations of sandstone, shale, and limestone, occurring at corresponding heights in hills many leagues distant from each other, with the same mineral characters and organic remains. It is clear that they have never been disturbed since the time of their deposition, except by such gradual movements as those by which large areas in Sweden and Greenland are now slowly and insensibly rising above or sinking below their former

level. The ancient limestone and shale also of the Canadian lake district before mentioned, are for the most part horizontal.

These facts are very important, as the more ancient rocks are usually much disturbed, and horizontality is a common character of newer strata. Similar exceptions, however, occur in regard to the more modern or tertiary formations which, in some places, as in the Alps, are not only vertical, but in a reversed position. These appearances accord best with the theory which teaches that, at all periods, some parts of the earth's crust have been convulsed by violent movements, which have been sometimes continued so long, or so often repeated, that the derangement has become excessive, while other spaces have escaped again and again, and have never once been visited by the same kind of movement. paroxysmal convulsions ever agitated simultaneously the entire crust of the earth, as some have imagined, the primary fossiliferous strata would nowhere have remained horizontal.

Had

Cambrian Group.-Below the Silurian strata in the region of the Cumberland lakes, in N. Wales, Cornwall, and other parts of Great Britain, there is a vast thickness of stratified rocks, for the most part slaty, and devoid of fossils. In some few places a few organic remains are detected specifically, and some of them generically, distinct

from those of the Silurian period. These rocks have been called Cambrian by Professor Sedgwick, because they are largely developed in N. Wales, where they attain a thickness of several thousand yards. They are chiefly formed of slaty sandstone and conglomerate, in the midst of which is a limestone containing shells and corals, as at Bala in Merionethshire. A slaty sandstone, forming the bottom of the Cambrian system in Snowdon, contains shells of the family Brachiopoda, and a few zoophytes.*

In some of the slate rocks of Cornwall, referred by Professor Sedgwick to the Cambrian group, cephalopoda of a very peculiar structure, called Endosiphonites, have been detected, a form which appears not yet to have been observed in the Silurian formation. The siphuncle in this shell is

Fig. 283.

16

Endosiphonites carinatus, Ansted.+ Cambrian strata, Cornwall.

* Phillips's Geology, vol. i. p. 129.

vol. xcvii.

Lardner's Cyclop.

+ Camb. Phil. Trans. vol. vi. pl. 8. fig. 2.

ventral, in which character it differs both from ammonite, in which it is dorsal, and from nautilus, in which it is central, or nearly central.

Although the Cambrian group can scarcely yet be said to be established on the evidence of a distinct assemblage of fossils, yet so great is the thickness of strata beneath the lowest of the welldetermined Silurian rocks, all of a date posterior to the creation of organic beings, that we may reasonably expect to be able to divide the primary fossiliferous strata into two groups.

467

CHAPTER XXIII.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocksTest by superposition and intrusion- By alteration of rocks in contact Test by organic remains - Test of age by mineral character Test by included fragments Volcanic rocks of the Recent and Newer Pliocene periods Miocene Eocene- Cretaceous Oolitic- New Red sandstone period-Carboniferous- Old Red sandstone periodSilurian- Upper and lower Cambrian periods — Relative ages of intrusive traps.

HAVING referred the sedimentary strata to a long succession of geological periods, we have next to consider how far the volcanic formations can be classed in a similar chronological order. The tests of relative age in this class of rocks are four: 1st, superposition and intrusion, with or without alteration of the rocks in contact; 2d, organic remains; 3d, mineral character; 4th, included fragments of older rocks.

Test by superposition, &c. If a volcanic rock rest upon an aqueous deposit, the former must be the newest of the two, but the like rule does not hold good where the aqueous formation rests upon the volcanic, for we have already seen (p. 181.)