In his account of the volcanoes of Iceland, Thoroddsen describes two types of acid lavas which have been extruded in post-Glacial, prehistoric times. First are the lavas of the Torfajökull district, of the usual nature of acid flows, building up a compact mass of bluish-grey rock with an outer casing of obsidian and pumice. These lavas have been poured out in the post-Tertiary country, but the second type of stream is found most frequently in the older parts of the island, which otherwise have witnessed no volcanic eruptions since Tertiary times. This type of flow results from eruptions of a peculiar character in which “vast outflows of halfmelted and unmelted masses of liparite, poured out from cauldronshaped depressions, stretch down into the lowlands” (1, p. 503). The best example of these “Liparitische Bloekströme” occurs in the Lodmundarfjord district, E. Iceland, and is especially described by Thoroddsen (2, pp. 159–161). In this paper the contention is put forward that the Lodmundarfjord blockstream is not a lava-flow but an unusual type of glacial moraine.

In a recent paper the writer described structures which he had found in the Frodingham Ironstone of Lincolnshire. The claim that these structures are fossil fungi is one of more than usual interest, for fossil fungi have not hitherto been recorded from the Jurassic rocks, and, further, the decomposition established by this fungus must have been carried out under marine conditions. Since the publication of this paper criticisms as to the conclusions contained in it have not been wanting, and it is proposed in the present paper to deal with these criticisms and further to furnish a few additional data to supplement those that have already been given.

During the last ten years I have published a series of memoirs on different questions concerning the morphology and biology of the Rugose Corals. In these memoirs I have elucidated the ground-form of the polyparium of the solitary Rugosa, the mode of its attachment to the substratum, the origin of the characteristic arrangement of the septa, and the origin of the fossuloe in dependence on this form and attachment. I have established the connexion between the form of the polyparium of the Rugosa (including that of the fossulse) and the life of these corals in definite environmental conditions.

During the past two years the teaching collection of rocks in the Geological Department of the Imperial College has been underre-arrangement, and in the course of the work the writer has had occasion to consider very carefully the principles on which igneous rocks should be classified. He is aware that in the present state of our knowledge any such classification must be tentative and experimental, and should be judged according to its general convenience, both for teaching purposes and for understanding the various problems that arise from petrological studies. The following article summarizes the conclusions arrived at, and while the writer must be held entirely responsible for all expression of opinion, he wishes to acknowledge his gratitude and indebtedness to Professor Watts and Dr. Evans, both of whom, in numerous discussions, have freely offered suggestions and criticism which have proved to be of the greatest value.