126 PROCEEDINGS OE THE GEOLOGICAL SOCIETY [May 1894, . 



with considerable detail, the Author observing tbat variolite has 

 Deen defined as a ' devitrified spherulitic tachylyte, typically coarse 

 in structure,' though she is disposed to place her own interpretation 

 upon the last phrase. The phenomena observed correspond in many 

 respects with those of the Durance and the Fichtelgebirge, as noted 

 by the Authors quoted on the preceding page. As regards the age 

 and associations of these variolite-bearing rocks of the Lleyn, there 

 are volcanic rocks including lava-flows, and fragmental masses 

 both of fine ash and coarse agglomerate. These are found in com- 

 pany with limestones, grits, and other rocks, which are possibly 

 of sedimentary origin. Without suggesting any particular age, 

 Miss Eaisin considers these variolite-bearing rocks to be of high 

 antiquity. 



The neighbourhood of Ffestiniog is so well known to tourists, 

 and especially to artists, from the picturesqueness of its scenery, 

 that some account of its geological structure might seem to possess 

 a popular as well as a scientific interest. The paper by Messrs. 

 Jennings and Williams on Manod and the Moelwyns is a contri- 

 bution in this direction. The area described by these Authors 

 forms part of the northern ring of the Merionethshire anticlinal ; the 

 Upper Cambrian strata dip under the mountains which give the title 

 to this paper, and are there overlain by ashes and slates of Arenig 

 age. Apart from the palaeontological and stratigraphical evidence 

 offered by the Authors with respect to the Lower Palaeozoic rocks of 

 this district, they show the intrusive nature of the great crystalline 

 mass known as the syenite or granitite of Tan-y-Grisiau, and to its 

 intrusion they attribute the metamorphism of the surrounding 

 rocks. This, then, is another case of contact-metamorphism, though 

 it does not appear that any of the characteristic minerals are 

 largely developed. Round the junctions the sedimentary rock is 

 altered into a compact Jwrnfels, and in some places it is very diffi- 

 cult to distinguish the altered rock from the finely crystalline edge of 

 the granitite. Judging from the map which illustrates this paper, 

 the surface-exposure of the granitite lies wholly within Tremadoc 

 rocks ; but, in spite of what the Authors call the intensity of the 

 metamorphism, there is a striking absence of distinct crystals. The 

 brown mica so common in areas of contact-alteration is absent 

 throughout. In some cases, where spots are developed, these have a 

 tendency to quadrilateral form, and may, in the Authors' opinion, be 

 embryo crystals of andalusite ; but there is nothing resembling the 

 rocks of the altered region about Skiddaw. This deficiency may be 



