•644 CAEBONIPEKOTJS DOLERITES AND TUEES. [Nov. 1 894, 



patient and exhaustive industry the field-relations of the rocks. 

 He felt that it would probably be feasible to establish two distinct 

 petrographic types among the dolerites or diabases, one characteristic 

 of the contemporaneous flows, the other of the sills ; and that not 

 improbably some distinction might be made out, between the frag- 

 mental material which consolidated in the vents, and that which was 

 discharged over the sea-bottom. There were probably many dis- 

 tinct volcanic platforms in the district, but to determine their 

 succession accurately it would be necessary to work out in detail 

 the stratigraphy of the Carboniferous Limestone. Obviously an ex- 

 ceedingly interesting chapter in the volcanic history of this country 

 was recorded in Derbyshire, and he trusted that the Author, living 

 as he did in the county, would undertake to decipher it. 



Mr. W. "W. Watts congratulated the Author both on the work he 

 had done and on the way in which he had presented it to the Society. 

 He thought that the perfect ophitic structure shown in some of the 

 sections would hardly be developed in a lava-stream, but only in a 

 sill. "With regard to the identification of iddingsite, he had found 

 at least two varieties of brown pseudomorphs of olivine in similar 

 rocks, some of which were uniaxial while others were biaxial. 



Dr. Johnston-La vis complimented the Author on working out so 

 thoroughly not only the massive rocks, but also the tuffs, which were 

 frequently neglected. He attributed the absence of olivine either to 

 the circumstance that the mineral has not individualized, or to the 

 facility with which it cracks and breaks up by a change of tem- 

 perature, so that the fine fragments might be easily overlooked in 

 old altered rocks like these. He would like to know whether the 

 calcareo-igneous breccia was near the bottom of the deposits, as it 

 would most likely occur at the first explosive disruption at the 

 initiation of another cone. At any rate, he should expect it to be 

 associated with pumice, and he thought that one specimen shown 

 by the Author was undoubtedly an old altered pumice. 



The Author thanked the speakers for the manner in which 

 they had received the paper. The interesting way in which Sir 

 Archibald Geikie had dealt with the subject would certainly induce 

 him to continue the work. In answer to Mr. Watts, the Peak Forest 

 rock referred to by Sir A. Geikie is an ophitic dolerite. In reply to 

 Dr. Johnston -Lavis, in one place at least, namely, in Ember Lane, 

 the tuff is more calcareous at the bottom of the exposed part of the 

 deposit than at the top. He did not think that the pseudomorph 

 of olivine was a surrounding of olivine by mica, as the alteration 

 proceeded along the cracks until the whole of the olivine was 

 replaced. 



