﻿Vol, 64.] BASIC INIEUSION OF BAETESTEEE. 511 



Fig. 3. Olivine-basalfc (46), near the base of the southern margin of the 

 dyke : X about 25. 



The whole or parts of three well-formed and very fresh olivine- 

 crystals are seen. Grains of magnetite and of very fresh augite with 

 felspar-laths make up the rest of the section. (See p. 508.) 



4. Basalt (11), near the southern margin of the dyke : X about 15. 



No olivine is present in this section, and the augite-crystals, which 

 are not very clearly seen in the photograph, are characterized by the 

 corrosion of their borders, and by the separation of magnetite, giving 

 them a speckled appearance. (See p. 507.) 



5. Basalt with doleritic patches (31), near the base of the northern 



margin of the dyke : X about 15. 



An irregularly-rounded patch of dolerite is seen enclosed in the 

 basalt, much magnetite and augite being aggregated along the line 

 of junction. (See p. 509.) 



6. Altered Old Eed Sandstone, close to the northern margin of the 



dyke : X about 25. (See p. 602.) 



The clear areas are patches of quartz with corroded borders. 

 Numerous needles of recrystallized felspar are also visible. 



Discussion. 



The Peesident thought that the explanation offered by the Author 

 was correct ; it was applicable to all the composite dykes that 

 had come under his own observation. 



Dr. Hatch congratulated the Author on having so well described 

 an excellent example of a multiple dyke. Judging by the thin 

 sections thrown on the screen, he thought that the micro-structure 

 of the Bartestree rocks was coniparable to that of some of the 

 weU-known types of Carboniferous dolerite occurring in the 

 Midland Yalley of Scotland. He asked the Author what was 

 the distinction that he made between ' dolerite ' and ' basalt ' in 

 the present case. He (the speaker) deprecated the application of 

 the terms * basalt ' aud ' andesite,' which should be reserved for 

 basic and intermediate lava-types, to rocks of undoubted hypabyssal 

 character (such as those of Bartestree), of which the proper 

 equivalent designations were dolerite and porphyrite respectively. 



Mr. J. Y. Elsdek said that he had been particularly interested in 

 the Author's conclusion that the analcime recognized in some parts 

 of this dyke was of primary origin. Most of the admitted occur- 

 rences of primary analcime had been in connexion with rocks 

 consolidated under considerable pressure. As some of the Author's 

 specimens contained glassy patches, he would like to ask whether 

 the analcime might not be altered glass. 



The AuTHOE said, in reply to Dr. Hatch, that he had used the 

 term ' dolerite ' to denote a relatively coarse-grained rock, and 

 the term ' basalt ' to denote one of relatively fine grain. In reply 

 to Mr. Elsden, he stated that the analcime was found in the freshest 

 of the doleritic rocks ; and, in reply to a question asked by the 

 President, that ilmenite was present in certain parts of the dyke. 



