Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 643 



Fig. 4. Augite entirely embedded in the calcite-cement of the Hopton tuff, 

 Outcrop 53. It extinguishes at an angle of 26° with the cracks which 

 run parallel to its length. On either side of it are yellow lapilli 

 which contain felspars, and are isotropic. 



Fig. 5. Ashover tuff, Outcrop 59. The lapilli contain no crystals. The tuff 

 is fine-grained, and the cement consists of smaller lapilli with a little 

 calcite. 



Fig. 6. Part of a spheroidal block in Kniveton tuff, Outcrop 56, showing three 

 adjacent amygdaloids separated by thin walls consisting of felspar, 

 magnetite, and interstitial matter. In the narrowest portions the 

 felspars are arranged tangentially to the boundary of the wall. 



Discussion. 



Sir Archibald Geikie, in complimenting the Author on the 

 completion of a laborious and valuable piece of work, alluded to 

 some of the striking points of resemblance between the microscopic 

 structures of the Derbyshire volcanic rocks and those of the Car- 

 bonifercms series in Central Scotland. It appeared to him, however, 

 that the petrography of the region would not be adequately under- 

 stood until the history of the volcanic phenomena had been investi- 

 gated in the field with far more minuteness than had yet been 

 attempted. Having recently for the first time visited the ' toadstone ' 

 area, he was able fully to confirm the suspicion which he had 

 long entertained, that the story of these volcanic rocks was far 

 more varied and interesting than had been supposed. He had often 

 wondered why none of the vents of discharge had been detected in 

 a region so deeply trenched with valleys as Derbyshire. But in the 

 course of a few days he succeeded in discovering at least six of these 

 vents. They are filled with coarse unstratified agglomerate, and 

 are sometimes traversed by dykes or veins of dolerite or basalt. 

 As admirable examples, he described two such vents at Grange Mill, 

 west of Matlock Bath, where they rise through the limestones and 

 are flanked with a band of finely-bedded tuff, which may mark the 

 material ejected from them. Vents are found from the extreme 

 north to the extreme south of the limestone area, and even traverse 

 the Yoredale rocks. 



While many of the toadstones are true lava-streams which, either 

 with or without fragmental accompaniments, were poured out over 

 the floor of the Carboniferous Limestone sea, he felt tolerably certain 

 that some of them are intrusive sills. In internal structure they 

 present a close resemblance to the usual type of Carboniferous sill in 

 the basin of the Firth of Forth. At one locality near Peak Forest he 

 found that the limestone overlying one of these sheets is marmorized 

 near the contact. Again, in Tideswell Dale, as was well known, a 

 band of clay underlying another similar rock has been made 

 columnar to a depth of 9 feet. 



In his rapid traverses of the ground, the speaker had the great 

 advantage of the intimate local knowledge of the Author of the 

 paper, who had kindly guided him to the sections which he specially 

 selected as most likely to throw light on the volcanic history of the 

 region. He had urged Mr. Arnold-Bemrose to take up with the same 



Q. J. G. S. No. 200. 2 y 



