Reports and Proceedings — Geological Society of London. 283 



for the most part, of volcanic origin. These may be divided into 

 a Rhyolitic Series below and an Ashj^ Series above. The Rhyolitic 

 Series is formed chiefly of lava-flows ; the Ashy Series consists mainly 

 of volcanic ashes and slates, the ashes predominating below and the 

 slates above. JDidymograptus hifidus occurs near the base of the Ashy 

 Series, D. Murchisoni in the upper part. 



There is not sufficient evidence to show whether the Rhyolitic 

 Series is conformable or unconformable with the Tremadoc Beds ; 

 but. since it lies between the Bictyonema Zone and the Didymograptus 

 hifidus Zone, it belongs presumably either to the Lower or to the 

 Middle Arenig. The Ashy Series extends from the Upper Arenig 

 to the Llandeilo. 



There are many intrusive masses of igneous rock, the largest being 

 that which forms the greater part of Mynydd Gader itself. This 

 intrusion is L-shaped in section, consisting of a horizontal limb 

 which cuts across the beds, and of a descending limb which lies 

 approximately in the bedding. 



The petrology of the igneous rocks, not only of Mynydd Gader, but 

 also of the area described in the previous paper, is dealt with. 



The intrusive rocks ai'e mostly dolerites, consisting chiefly of augite 

 and plagioclase (labradorite-andesine), without olivine or rhombic 

 pyroxenes. Epidote is very abundant as an alteration product. 

 Contemporaneous veins are met with at one locality. Small intrusions 

 of eurite occur just beyond the limits of the map. 



The rhyolites are sometimes compact, sometimes banded, and 

 sometimes nodular. Analyses of two specimens, the one compact 

 and the other nodular, show that the percentage of soda is high. 



The tuffs or ashes vary considerably in character. Some consist 

 of a fine matrix with numerous scattered angular fragments of all 

 -sizes up to 18 inches; the fragments are mostly rhyolitic, but pieces 

 of slate and grit also occur. In others the fragments are smaller, and 

 some are so finely laminated that on freshly broken surfaces the 

 texture appears perfectly uniform, but on weathered surfaces the 

 lamination is often quite distinct. 



Andesitic lavas occur at two horizons, in the Upper Lingtda Flags 

 and in the Llandeilo Series. The latter are often highly amygdaloidal. 



The Rhyolitic Series appears to be older than the main mass of 

 volcanic rocks in the Arenig area, but it may be contemporaneous 

 with the Calymene Ashes of that district. It is probably of 

 approximately the same age as the volcanic series of Skomer Island, 

 and the fact that in both places the rhyolitic rocks are soda-rhyolites 

 is of considerable interest. 



2. "Insect-Remains from the Midland and South-Eastern Coal- 

 fields." By Herbert Bolton, F.R.S.E., F.G.S., Director of the Bristol 

 Museum. 



The writer describes a series of three insect-wings obtained by 

 Dr. L. Moysey, F.G.S., from the Shipley Clay -pit near Ilkeston 

 (Derbyshire), and a blattoid wing and three fragments from the 

 borings of the Kent Coal Concessions Company, Ltd., in East Kent. 



The first series of insect-wings occur in greyish-brown ironstone 



