Freshwater Limestone in Forfarshire, Sgc. 79 



Plants. — Among the vegetable remains of the limestone is one which proves 

 to be strictly analogous to the Gyrogonite of the ancient freshwater forma- 

 tions, and which therefore is a subject of much geological interest ; particu- 

 larly as many differences of opinion existed among naturalists for a long time 

 concerning the nature of that fossil : and the doubts of some, whether it be 

 really the seed-vessel of a Chara, have not yet been set at rest. In order, how- 

 ever, not to interrupt with too much detail the course of the present paper, I 

 shall treat of the Gyrogonites, and of the stems of Charae which accompany 

 them, in an Appendix. [See App. I.] 



On the Origin of Shell-marl, and on the Circumstances which favour its 



Production. 



Though frequently not a trace of organization can be discovered in the 

 marl of Forfarshire, yet not a doubt can reasonably be entertained that it is 

 derived from freshwater shells *, corresponding in the occasionally total obli- 

 teration of its original shelly texture with the white pulverulent marl of Glen- 

 tilt, described by Dr. MacCuUoch, and shown by him to result from land-tes- 

 tacea. (See Geol. Trans, vol. iii. p. 316.) In order, therefore, to explain the 

 occurrence of the marl in the lakes of Forfarshire, it is only necessary to ac- 

 count for the accumulation of freshwater shells in those lakes. 



In a paper on the Shell-marl of Scotland, (Geol. Trans. 1st series, vol. iv. 

 p. 308.) Mr. Warburton has already remarked " that since shell-marl is not 

 equally common in all the Scotch lakes, it may be worth inquiring what pecu- 

 liar circumstances have favoured its production ; whether calcareous salts are 

 particularly abundant in the waters that yield it ; and what are the rocks from 

 which these waters spring, or which form the sides of the containing basins." 

 I shall endeavour to throw some light on these leading topics of inquiry. 



On viewing the principal deposits of marl in Forfarshire and in the eastern 

 part of the adjoining county of Perth, it will be found that they do not occur 

 in the districts consisting of granite, gneiss, mica-slate, clay-slate or grey- 

 wacke, although those districts contain many lakes ; but that marl-lakes are 

 frequent in the inferior sandstone, and are not uncommon in the old red-sand- 

 stone, and that to these two formations they appear to be strictly confined f. 



One of the remarkable features in the geology of the county of Forfar is 



* The growth of Charaj may also have contributed to the secretion of calcareous matter ; their 

 stems containing, as will be found stated in the Appendix No. I., an abundance of carbonate of 

 lime. 



t See the accompanying Map. Plate XI. 



