GEOLOGY. 497 



rocks. The principal secondary rocks are sandstone, limestone, 

 and trap, arranged in various positions, and associated with other 

 rocks. Professor Jameson thus enumerates them in the order 

 of their relative position : 



" 1. First Sandstone, or Old Red Sandstone Formation. This is 

 a reddish-brown sandstone, principally composed of particles of 

 quartz, either without the ground, or connected together by a basis 

 or ground of iron-shot clay. It passes into greywacke, as on the 

 coast of Galloway. It rests upon the rocks of the transition class. 



" 2. First Secondary Limestone, or Mountain Limestone is a 

 compact bluish-gray limestone, full of encrinites, corals, and shells. 

 Often contains caverns, and sometimes alternates with the sandstone, 

 slate-clay, and other rocks of the coal formation. It lies immediately 

 on the old red sandstone. 



" 3. Coal Formation. This is an alternation of gray and white 

 sandstone, bituminous shale and slate clay, clay ironstone, limestone, 

 and coal. The whole together form a group or set of rocks, termed 

 the coal formation. It rests on the mountain limestone. 



te 4. Second Secondary Limestone, or Magnesian Limestone of 

 Geologists This formation, as it appears in England, is generally 

 a granular, sandy, and glimmering limestone, which contains a con- 

 siderable portion of carbonate of magnesia. It occasionally contains 

 gypsum and rock salt. It lies immediately over or above the coal 

 formation. 



" 4. Second Sandstone, or New Red Sandstone Formation This 

 sandstone is principally composed of particles of quartz, set in a red- 

 dish-brown clayey basis or ground. It is looser in its nature than 

 the old red sandstone, and its colour wants the bluish tint which oc- 

 curs in the old red sandstone. It is sometimes conglomerated, par- 

 ticularly where near the magnesian limestone, when it contains frag- 

 ments of the subjacent strata. It abounds in beds of red and blue 

 marl and clay, and in these there are occasionally imbedded masses 

 and beds of gypsum, and rock salt. It is here, and in the magne- 

 sian limestone formation, that the greatest masses of rock salt are 

 met with, and it is in these formations of the secondary series that 

 the principal salt mines are situated. It rests immediately on the 

 second secondary or magnesian limestone. 



C( 5. Third Secondary Limestone, or the Oolite or Shell Limestone 

 Formation, or Jura Formation. The lower members of this forma- 

 tion are blue, gray, and white slaty limestone, with blue slaty marl, 

 and clay, in which are variously shaped masses of chert. These are 

 known under the name Lias. Above these, still in this forma- 



VOL. II. I 1 



