Many of the common things that occur to us in our walks and rides near home, and which occasionally are noted in our memorandum-books on account of the clearness of their features, or for the sake of some occasional peculiarities in the details observable in them, may not only often prove as useful exemplifications of natural processes and phenomena as facts and instances brought from any distant part of the world, but may bear repeated description and fresh demonstration for the sake of geological enquirers, such as many of the correspondents of The Geologist must necessarily be.

Since some pages of your valuable magazine are occasionally devoted to the description and illustration of “Gems from Private Collections,” allow me to offer a drawing and some observations on Spirifera convoluta, Phillips, one of the most remarkable of our British carboniferous spirifera, and which will serve at the same time as a small supplement to the paper I published in the eleventh number of The Geologist.

At page 35 of my “Monograph” I described Sp. convoluta as a distinct species, but subsequently I thought that it might, perhaps, be a very transverse or exceptional condition or variety of Sp. bisulscata, Sowerby.

Experience teaches us that individuals of the same species may assume great differences in shape; that a Spirifera, for example, which in its normal condition is about as wide as long, may at times exceed these proportions very considerably in one or the other direction, and that we are often too prone to seize upon similar variations as pretexts for the creation of so termed species.

For some years past it has been a commonly received doctrine among continental geologists that lava-streams could not consolidate on slopes or declivities of more than five degrees. When, then, solid lavas were found in various volcanic mountains at high angles, other sources of origin than the mere outflow from their respective craters had to be sought; and thus arose the theoretical idea of the formation of the cone or crater at a late period of the volcano's existence—after for ages numerous lava-streams had issued from it and had become consolidated one over the other into stony beds on successive flat plains—by the uprise and bursting of a vast dome or bubble; and such was called a “crater of elevation”.