Anyone who studies the relation of the rocks of the Lizard Peninsula to those immediately to the north must realize that Porthallow, the little village in a sheltered cove near St. Keverne, is a critical locality in the problem, and one indeed that may hold the key to the solution of the relative ages of the rocks on either side of the sinuous line called the Lizard Boundary. Porthallow is at the eastern extremity of that line. The western extremity is on Polurrian Beach, near Mullion, where a very clear fault with a breccia about 22 feet thick separates two totally distinct formations, sandy and argillaceous killas on the north from hornblendegneiss on the south. At Porthallow, however, although there are cliffs on either side of the cove washed by the sea, the geology is far from having the textbook clearness of Polurrian Beach and there is room for much difference of opinion. Text-fig. 1 is a sketch-map of the locality based on the Ordnance Survey 25 in. to the mile map, and first the main features of the geology will be described. For the neighbouring country the reader is referred to the Survey 1 in. map (Sheet 359) and to the 6 in., Sheet lxxxi N.E.

In several papers by the present author alone or in collaboration with Dr. J. E. Hemingway, mention has been made of the abundance of the titanium minerals, rutile, anatase, and brookite, in the Middle Jurassic sediments of Yorkshire. One of these papers dealt with brookite crystals from the Dogger, and the present one will describe briefly the types of rutile crystals from the same formation; partly because these show interesting peculiarities, and partly, as a subsidiary object, because it has appeared during this investigation that the descriptions given in most of the books of reference of sedimentary rutiles are somewhat inadequate, several striking features found to be widely prevalent not being mentioned in any of them, while other points are dealt with very briefly by most authors. A beginner in sedimentary petrography, confronted with a heavy mineral concentrate of this kind, might find considerable difficulty in identifying the crystals of rutile.

During the summer we paid two visits to the site of a sewer trench that was being dug from Aylesbury along the London Road towards Tring. At the time of our visits excavation was being carried on in the village of Aston Clinton.

Two publications by Grabau, which will be treated in the following pages, have led to the ensuing considerations.

In his opinion world-wide advance and retreat of the sea are simply explained by vertical movements of the sea-level. About the possible cause of these movements he makes some very brief remarks, as we shall see later. It is, however, quite possible that the explanation must be sought in simultaneous but opposed movement of continents and sea-level. In trying to obtain an insight into the cause of these movements, rhythms of an entirely different sort come up for discussion, viz. phases of diastrophism and magmatic cycles. And then a further question presents itself: can these different phenomena be connected, and thus give an insight into certain rhythmical processes in the deeper parts of the earth ? I, personally, am convinced that we must answer this question in the affirmative. Stille, Joly, Holmes, and Bucher have reached a similar conclusion by different ways. Meanwhile, the following pages are only intended to shed some light on the different aspects of the problem, and I shall restrict myself to outlining the questions that demand further study.

The sub-genus Lycophoria was founded by Lahusen (1886, p. 221), and referred to the Strophomenidae, the only species described, Lycophoria nucella (Dalman, 1828, p. 130) being the genotype. Part of the original diagnosis runs: “Klappen convex; Rückenklappe stärker gewölbt, am Stirnrande ein breiter flacher Sinus. In der Bauchklappe eine kleine dreiseitige Area mit dreieckiger offener Deltidialspalte. Schlosszähne von zwei divergirenden Zahnplatten gestützt, die in ihrer Fortsetzung am Grunde der Schale eine längliche, durch eine kurze Mittelleiste getheilte Muskelgrube umgrenzen. In der Rückenklappe ein starker, hakenförmiger Cruralfortsatz, innen durch eine Längsfurche getheilt und auf der Rückseite mit einem kleinen, zahnartigen Schlossfortsatz verbunden” (Lahusen, 1886, p. 221). Lycophoria has also been discussed by Hall and Clarke (1894, p. 230), by Kozlowski (1929, p. 131), who placed it with the Porambonitidae in the Telotremata, and by Schuchert and Cooper (1932, pp. 105–6), who regarded it as the sole representative of a new family of the Orthacea. The close external similarity to Porambonites and the fact that Lycophoria had also been referred to the Pentameracea, led the writer to investigate Lahusen's genus in a search for possible ancestors of the pentamerids.