OUNHN OF LIMESTONES. 29 
acting along a coast-line. Similar effects might be produced 
locally by slight depression. 
The evidence of this shallower-water formation of the lime- 
stones, is on the whole stratigraphical rather than palseontological, 
and it must not be forgotten that there are areas, such as Lyine 
Regis and Rugby, where the limestones at the base of the Lias 
are well-developed, and where we have no evidence for or against 
the proximity of land. Again on the eastern borders of the 
Bristol coal-field, where the Lias comes occasionally into direct 
contact with the Palaeozoic rocks, limestones are not prominently 
developed. The occurrence of Insects is suggestive of the 
proximity of land, although they may have been blown out to 
sea, or floated down rivers. Occasional small masses of lignite 
are found, and Gasteropoda as well as Saurians, are fairly abundant 
in some of the limestones. 
The questions that arise concerning the limestones are to what 
extent the calcareous mud was deposited more or less mechanically 
or precipitated chemically ; and to what extent the calcareous 
matter was derived by mechanical abrasion, and by the accumu- 
lation and decay of organisms and of material voided by them. 
The stratigraphical evidence shows that there must have been 
considerable though variable accumulations of highly calcareous 
mud. The limestones and shales of the Lower Lias occur in 
more or less rapid alternation. Some of the limestones, found in 
persistent layers, present markedly irregular surfaces, being more or 
less protuberant or nodular, while the separating bands of clay or 
shale contain nodules and lenticular masses of limestone. It 
seems most reasonable to believe that in these cases the calcareous 
matter segregated from the more argillaceous portions of the mud. 
The same would have been the case with the nodular limestones 
01 cement-stones and septaria that occur in the mass of the clays, 
and in particular with those Upper Lias nodules that enclose 
remains of Fishes, &c.* In other cases where the limestones occur 
in comparatively even bands, they are usually more or less striped 
and laminated, and they pass by insensible gradations into calca- 
reous shale. Such is the case with the Insect Limestones and 
associated strata in the Lower Lias, and their sedimentary origin 
can hardly be doubted. 
This subject of the more or less detrital origin of the Lias 
limestones has occupied my attention during the past eight years, 
but I was unaware, until the above remarks were written, that 
Prof. Sollas, had in 1879, published some notes on the same 
subject. These notes were printed in a paper dealing with the 
Silurian district of Rhymney, near Cardiff, t After stating the- 
evidence on which he concludes that the cornstones of the Old 
Red Sandstone originated from mechanically-formed sediments, 
he points out that there are other instances in which limestones 
have been derived " from sediments which have been carried in 
suspension and strewn out in deposits, in just the same fashion as 
* See De la Beche, Researches in Theoretical Geology, pp. 95, &c. 
t See Quart. Journ. Geol. Soc., vol. xxxv. p. 492 ; see also vol. xxxix. p. 614. 
