1&25.] (Jeulugical Society. 311 



The rock marl described in this communication is an extremely 

 compact limestone, in part of a crystalline structure, and tra- 

 versed by numerous irregular tubes or cavities. 



As a principal part of its geological interest is derived from 

 its recent origin, the author has drawn a brief sketch of the 

 physical structure of the county of Forfar, in order to explain 

 distinctly its position. 



Those strata are also enumerated in which limestone is found, 

 and its remarkable scarcity in Forfarshire pointed out. 



The districts to which shell marl is confined are next consi- 

 dered, and it appears that deposits of this nature are accumu- 

 lated only in lakes in two formations, viz. the inferior or transition 

 sandstone, and the old red sandstone. 



The Bakie Loch, in which the rock marl occurs, lies in a hol- 

 low in sand and gravel. This gravel consists of the broken and 

 rounded masses of the primitive rocks of the Grampians, which 

 are heaped in large quantities upon the old red sandstone in the 

 valley of Strathmore. 



The succession of the deposits of sand, shell marl, and rock 

 marl, in the lake of the Bakie now drained, is then described. 

 The shells and plants enclosed in the rock are the same as those 

 in the soft shell marl, and are ail still living in the waters on the 

 spot. Among the plants are the stems and seed vessels of 

 Charai, the latter being fossilized in such a manner as to present 

 a perfect analogy to the gyrogonite of the ancient freshwater 

 formations. 



Mr. Lyell then considers the probable origin of the rock marl, 

 which appears to be derived from the subjacent shell marl, 

 through whicli springs ascend, charged with carbonic acid. 



Some remarks are next offered on the shell marl of Forfarshire, 

 and some which the author has examined near Romsey, in 

 Hampshire, is described. The subjects of chief interest with 

 regard to the shell marl are, its slow growth, the small propor- 

 tion of full grown shells, which are found in it in Forfarshire, 

 the greater rapidity of its growth in the vicinity of springs, its 

 abundance in a part of Scotland in which limestone is very rare, 

 and its scarcity in the calcareous districts of England. 



The question is then considered whether the shell marl be 

 exclusively derived from the exuviae of testacea, and the various 

 arguments for and against this hypothesis are entered into. 



in conclusion Mr. Lyell takes a general view of the analogy 

 between the ancient and modern freshwater formations. 



Both of these may be described generally as consisting of 

 thin beds of calcareous, argillaceous, and arenaceous marls, 

 together with strata of sand and clay, to which the consolidated 

 beds bear upon the whole but a small proportion. 



The shells and plants contained in both are referable to the 

 same genera. 



