82 me. e. n. worth on the igneotjs constituents of the 



Discussion. 



The President said that the question -whence the materials were 

 derived was one of great- interest, and must be worked out by careful 

 determination of the specimens. 



Prof. BoNNEY observed that the subject was one difficult to dis- 

 cuss, requiring great caution. But few specimens were shown, which 

 added to the difficulty ; we required to see those from the breccia 

 and the in-siiu rocks together. He was inclined to think the Author 

 right in his main contentions, and knew that he was an enthusiastic 

 worker. There was certainly Dartmoor granite in the breccia : those 

 rocks also with large crystals in a more compact matrix might repre- 

 sent more rapidly consolidated portions of the same material. He 

 was glad a Devonshire geologist had taken up the subject. The 

 breccias indicated unmistakably the proximity of high land at the 

 time of their deposition. 



Dr. Geikie had been over part of the ground, and was unable to 

 see the connexion between the composition of the breccias and the 

 supposed occurrence of a volcano on the site of Dartmoor. The 

 materials had not come from very far. During Triassic times there 

 was undoubted volcanic activity from many points of eruption, so 

 that there was no necessity for bringing the basic volcanic rocks 

 from Dartmoor ; indeed, the presence of Dartmoor granite in the 

 breccias was unfavourable to the notion of volcanic rocks having 

 been derived from that source. 



Dr. Hice:s had no doubt that some of the fragments had been 

 derived from rocks such as are known to occur in Dartmoor ; but 

 that need not necessarily indicate more than that there was a pre- 

 existing ridge of such rocks in that area. 



Mr. HuDLESTON had arrived at much the same conclusion as Dr. 

 Geikie, especinlly as to the source of most of the volcanic fragments. 

 If Dartmoor had been a volcanic pile at the time of the deposition 

 of the breccias, its bulk ought to have contributed more material in 

 proportion to the local sedimentary rocks than is the case. There 

 was no dj^:)no?n improbability in the Dartmoor granite having termi- 

 nated upwards, at one time, in volcanic rocks ; but the period was 

 probably not later than the close of the Carboniferous when the 

 great earth-movements took place. Most of this material would 

 have been removed before the accumulation of the Triassic breccias. 

 Hence he was prepared to accept the theory of a great Devonshire 

 volcano, but rather doubted the evidence of its existence in the 

 South-Devon breccias. 



Prof. Hughes inquired about the possibility of some of the frag- 

 ments having been thrown up by volcanic outbursts, from rocks not 

 then exposed at the surface. He had missed any reference to the 

 presence of harder volcanic material, such as is more or less seen to 

 be intercalated in the Devonian. 



Prof. JuDD said that the subject could only be attacked by a local 

 worker. He regretted the absence of the Author and that there 



