VIII. — On a recent Formation of Freshwater Limestone in Forfar shire ^ 

 and on some recent Deposits of Freshwater Marl ; with a Compa- 

 rison of recent ivith ancient Freshwater Formations ; and an Appen- 

 dix on the Gyrogonite or Seed-vessel of the Chara. 



By Charles LYELL, Esq., F.R., L., and G. S.S. 



[Read December 17th, 1824, and January 7th, 1825.] 



IHE freshwater limestone which it is now my chief object to describe, dif- 

 fers considerably in the circumstances under which it occurs, from any similar 

 production that has yet been noticed. In order that these circumstances may 

 be fully understood, I shall give a brief geological sketch of that part of the 

 county of Forfar in which the limestone has been discovered. 



The county may be regarded as divided into three principal districts ; that 

 of the Grampians, that of Strathmore, and that of the Sidlaw Hills. The gra- 

 nite of the Grampians skirts the northern frontiers of the county, and is fol- 

 lowed to the south by gneiss, mica-slate, clay-slate, and greywacke, each of 

 these formations in succession occupying a tract of considerable extent, and 

 each observing a north-east and south-west direction. Immediately on the 

 clay-slate rests a series of strata consisting of red and green shale, of gray 

 micaceous and slightly calcareous sandstone, and of red schistose sandstone, 

 this series being subject in all its parts to be interfered with by rocks of the 

 Trap family. This, which I shall call the series of inferior sandstone, has not 

 yet been ascertained to contain organic remains ; but in other leading respects 

 it resembles that assemblage of strata which in England, and particularly in 

 the counties of Hereford, Gloucester, and Monmouth, is so intimately connect- 

 ed with transition limestone. The inferior sandstone dips beneath the old 

 red-sandstone-conglomerate which flanks the Grampians, and emerges again 

 on the south side of the valley of Strathmore, there forming the Sidlaw Chain, 



The strata in that chain are disposed saddle-wise ; and along the axis or 

 anticlinal line of the saddle, which runs north-east and south-west, the oldest 

 members of the inferior sandstone series are displayed, lying almost horizon^ 



VOL. II. — SECOND SERIES. h 



