156 TROFS. LLOYD MORGAN AND REYNOLDS ON THE [May I904, 



EXPLANATION OF PLATES XYI & XVII. 



Plate XVI. 



Map to illustrate the distribution of the Carboniferous volcanic rocks in the 

 Bristol district, on the scale of 2 miles to the inch. 



Plate XVII. 



Fig. 1. Olivine-dolerite or basalt from Goblin Combe eastern exposure. 

 This shows several crystals of serpen tinized olivine, surrounded by 

 plagioclase-laths, the spaces between which are often occupied by 

 grains of augite. (See p. 152.) 



2. Olivine-basalt from Milton Hill, Weston (not in situ). A group of 



three crystals of altered olivine is seen, and with them much ferric 

 oxide is associated. 



3. Variolitic basalt from Spring Cove, Weston-super-Mare. Several 



varioles are seen, varying considerably in size ; also veins and amyg- 

 dules of calcite. (See p. 152.) 



4. Calcareous ash from Spring Cove, Weston-super-Mare (3 feet above 



the lava). This sbows small basaltic lapilli, embedded in an abundant 

 matrix of calcite. 



5. Calcareous ash from the extreme top of asb, Woodspring, exposure 



(b). This shows abundant lapilli of amygdaloidal basalt, embedded 

 in a matrix of calcite. (See p. 154.) 



6. Ashy oolitic limestone from Goblin Combe, eastern exposure. 



This shows abundant oolitic grains and well-marked lapilli, embedded 

 in a calcareous matrix. (See p. 155.) 



Discussion. 



Mr. A. Strahan said that he had listened with pleasure to the 

 careful description of these interesting rocks. The Authors had 

 added much to our knowledge ; for they had not only recorded 

 two new occurrences, but (what was more important) had deter- 

 mined that the volcanic rocks of the various localities occurred 

 at the same horizon in the Limestone. The object of the traverse 

 made by Sir Archibald Geikie and himself had been mainly to 

 complete the Cardiff sheet of the 1-inch Geological-Survey map; 

 and although the advisability of subdividing the Limestone and 

 determining the horizon of the volcanic rocks had been discussed, 

 no opportunity had arisen of carrying out the work. With the 

 assistance of Mr. Vaughan, the Authors appeared to have proved 

 that the tuffs all occurred at the same horizon. He (the speaker) 

 had suggested that the vent lay somewhere to the west, probably 

 under the Bristol Channel. Whether tuffs could have been so 

 evenly and widely distributed from one vent only was perhaps open 

 to doubt. He congratulated the Authors on the interesting results 

 of their investigation. 



The Chairman (Sir Archibald Geikie), alluding to the description 

 of the volcanic rocks given by Mr. Strahan and himself, to which 

 the Authors had referred, said that the examination of these rocks 

 had been undertaken by them, not with the view of making a 

 detailed study of the subject, but for the purpose of correcting the 



