SOME ANCIENT AND MODERN VOLCANIC ROCKS. 391 



temporaneous traps, or they are not recognized as such. One felsite 

 dyke in Auchenhew Hill is compared to rocks of the modern tra- 

 chydolerite group. In conclusion, the author urges the abandonment 

 of such terms as "porphyry" and " porphyrite " us generic terms, 

 and deprecates " the distinction now made between rocks of different 

 ages, when there is really no essential difference between them " 

 (p. 544). 



In 1872 Prof. Hull contributed notes on the microscopic structure 

 of some trap-rocks to the Government-Survey Memoir on Sheet 48 

 (Ireland). 



A paper by the same author " On the Microscopic Structure of 



the Limerick Carboniferous Trap-rocks (melaphyres)"* in the year 



1873, describes them as representatives of the more recent basalts, 



but points out a distinction between them and the Tertiary basalts 



of Antrim, thus : — 



Limerick. Carboniferous. 



-r, i i f Augite crystals, Triclinic felspar, Magnetite, 



a , e ' "',.-. 1 Olivine (pseudomorphs), Calcite and Chlorite 

 glass, containing , V 1 N r Ji 



D ' ° [ (accessories). 



Antrim. Tertiary. 

 Base, augi tic, con- f Triclinic felspar, Titan o-ferrite, Olivine (somc- 

 taining [ times), Chlorite (sometimes). 



In the Limerick traps he notes " the glassy felspathic base with 

 cells and tubes," " the small quantity of augite," " the abundant in- 

 fusion of chlorite," and " the abundance of calcite, also due to perco- 

 lation." The occurrence of hornblende with some of these augitic 

 rocks is also to be noticed. The author remarks in conclusion that 

 his observations tend rather to confirm the opinion of Mr. Forbes, 

 as expressed in his paper " Researches in British Mineralogy " f, viz. 

 that "when the geological epochs of the appearance of two or more in- 

 trusive or eruptive rocks are known to differ, these rocks will then 

 also be found to differ essentially in mineral constitution." This 

 paper drew forth a reply from Mr. Allport in the following month J, 

 entitled " Tertiary and Palaeozoic Trap-rocks." In it he asserts 

 that the glassy base " said to constitute a difference between the two 

 series of rocks (Limerick and Antrim), certainly occurs in the Antrim 

 basalts, and is common in many Carboniferous and Tertiary dole- 

 rites," and defines a glass base (dark under crossed prisms) as dis- 

 tinguished from a felspathic base (which exhibits double refraction). 

 After declining to admit "that a difference in the relative propor- 

 tions of any of the constituents is sufficient to indicate an essential 

 difference in the rocks in which they occur," the author remarks in 

 conclusion that " great differences certainly exist between augitic 

 lavas of different volcanic districts ; the same is true of those of the 

 older geological periods ; and similar differences occur between rocks 

 of various periods. It becomes evident, therefore, that a dissimi- 



* Geol. Mag. vol. x. pp. 153-161. t Phil. Mag. vol. xxxiv. p. 336. 



+ Geol. Mag. vol. x. p. 196-8. 



