466 Reviews — Dr. Rauff^s Palceospongiology. 



He gives as causes of formation — 



1. The downward creep of the surface and sub-surface soil. 



2. The occurrence of the terraces upon one precise geologic line, 

 the micaceous marls of the Middle Lias which come in below the 

 Eed Eock bed. The marls are porous and non-cohesive. On the 

 slippery slopes the soil must creep. The rain and rain-wash loosen 

 the light soil below and about the roots of the herbage and urge its 

 movement downward. Terraces from an incipient stage, like an 

 ordinary grass ridge, to minor and major terraced banks of varying 

 regularity of form can be traced. Below these marls are depths of 

 compact blue clay (the zone of Ammonites margaritaius if in near 

 contact with the marls). A little below the point where the marls 

 and clays meet is the line of water outflow. Along the line there 

 is constant removal of marl by chemical and mechanical solution. 

 The effect is the loosening and sliding of the land downwards and 

 outwards. This pressure is aided by the weight of the overlying 

 mass of rock, sometimes twenty-five feet in thickness. 



3. Free passage of water through the rock and marl is necessary, 

 for the Upper Lias clays have on the Oxfordshire terraced hills 

 either been wholly stripped from the hill top or pushed back by 

 atmospheric denudation. Eegularly terraced slopes are not found 

 on clay-covered hills ; the appearance of terraces is coincident with 

 the wearing away of the clay " roof." 



The amphitheatre form of terraced land is always a valley head. 

 The outflow of the stream — the valley-maker — marks ordinarily the 

 base of the amphitheatre. More frequently the terraces of the valley 

 head are small in slip and their curvature is broken. Such an instance 

 is Kenhill, near Shennington. An instance of greater regularity of 

 curvature and greater depth of step is the Beargarden, Banbury. 



From the Edge Hill escarpment a fork of the Horton vale runs 

 alongside Adsum Plantation, and makes what is known as Adsum 

 Hollow. The terraces sweep in regular curves along the high banks 

 of the stream, and where it joins the main vale to the north of 

 Horley the steps are so prominent as to give the name of Steps 

 Meadow to the ground. Gredenton Hill, on the Burton Darrett 

 range, is very regularly and beautifully terraced on three sides. 



The author does not attempt description of the Chalk hills or the 

 lynchets of Dorsetshire. The sandy marls of the Dorsetshire Inferior 

 Oolite have a composition approaching that of the micaceous marls 

 of the Midlands, and reasons like those brought forward will no 

 doubt prove their similar mode of formation. 



la E "V I S -w S. 



L — Pal^ospongiologie ; von Hermann Eauff. Erster Theil. 

 5^^ und 6*6 Lieferung. Mit 17 Tafeln und 27 Abbildungen im 

 Text. Palaeontographica. Band xl. Stuttgart, 1894, pp. 233-346. 



THE previously issued (1-4) parts of Dr. Eaufif's elaborate 

 Monograph on Fossil Sponges, which dealt with the general 

 history, structure, and classification of the group, have already been 



