SOME ANCIENT AND MODERN VOLCANIC ROCKS. 389 



Introduction. — The object of the present paper (submitted to the 

 Society with the permission of the Director -General of the Geolo- 

 gical Survey) is, by comparing the microscopic rock-structure of 

 several distinct groups of volcanic rocks, to gain an insight into the 

 original structure of some of the most ancient, and therefore pro- 

 bably the most altered, members of the volcanic series. My desire 

 has been mainly to note the relations of the component minerals to 

 one another, and thus glean facts in the history of the rocks which 

 they compose. For this purpose I have endeavoured by coloured 

 drawings to render as truthfully as possible the microscopic struc- 

 ture of most of the examples brought forward. 1 have to acknow- 

 ledge the very kind and willing assistance rendered by Mr. Allport 

 in helping me to take up and prosecute to a small degree this line 

 of research. 



I. History op the Subject*. 



Very little, comparatively, has as yet been done in England on 

 this subject ; but perhaps some of the most valuable work that has 

 as yet been effected in microscopic geology is the result of the labours 

 of our countrymen Sorby and Allport. 



In 1858, Sorby brought out his celebrated paper on the " Micro- 

 scopical Structure of Crystals " f . He describes the glass, stone, and 

 gas- or vapour- cavities in the minerals of the pitchstones of Arran, 

 the lavas of Vesuvius, and some of the basaltic rocks of Scotland. Rer 

 marking on the great alteration which has been effected in the 

 ancient trap-rocks during the course of ages, he nevertheless obr 

 serves (p. 479), "The characteristic structure of the minerals of 

 which ancient trappean rocks are composed is therefore so analogous 

 to, or even identical with, that of the constituents of modern lavas, 

 that the purely igneous origin of these ancient lavas appears to me 

 to be completely established ; but, at the same time, their present 

 aspect is often to a very great extent clue to the subsequent action 

 of water." With regard to rocks formed at a high temperature, the 

 author's chief conclusions are as follows: — "At one end of the 

 chain are erupted lavas, indicating as perfect and complete fusion as 

 the slags of furnaces ; and at the other end are simple quartz veins, 

 having a structure precisely analogous to that of crystals deposited 

 from water. Between these there is every connecting link ; and the 

 centra] link is granite " (p. 496). 



In 1867 an article appeared upon "the Microscope in Geology "jj:, 

 written by Mr. David Eorbes. The author shows how the different 

 minerals in volcanic rocks may be distinguished from each other by 

 means of the microscope, and gives figures illustrating the general 

 microscopic structure of the following rocks : — lavas of Etna, Vesu- 

 vius, and Tahiti ; pitchstone dyke of Arran ; Staffordshire basalt, 

 and "white horse" dykes; diorite from Chili ; uralite porphyry from 



* Reference will here only be made to those papers, or parts of papers, which 

 bear upon volcanic rocks. 



t Quart. Journ. Geol. Soc. vol. xiv. pp. 453-500. 

 I Popular Science Review, vol. vi. pp. 355-368. 



