A. R. Hunt — Petrological Nomenclature. 31 



was colder than that of the Coralline. The occurrence of this one 

 block, which was neither angular nor striated, and which might 

 have reached its resting-place in some other way, 1 does not seem 

 to me to justify the otherwise improbable hypothesis of floating 

 ice during any part of the Coralline Crag period. The affinities 

 of the Molluscan fauna of these deposits is clearly with the seas of 

 Southern Europe, or even, as Mr. Wood suggested, of the Azores, 

 rather than with those to the north of the Island. At the present 

 time there is a difference of not less than 10° F. between the average 

 yearly temperature, both of the atmosphere and of the ocean, in 

 the British and Mediterranean areas. 



VII. — Notes on Petrological Nomenclature : Schist, Slate, 



Phyllade, and Phyllite. 



By A. R. Hunt, M.A. 



IN 1891 Professor A. H. Green wrote an interesting notice of 

 two books, both of extreme value to the petrological student, 

 viz. Cole's Aids in Practical Geology and Hatch's Introduction 

 to the Study of Petrology. In the course of his review the 

 Professor remarks: " In dealing with the foliated rocks, the author 

 touches on the debated point of the ' true schists.' We are pretty 

 well used to this phrase, and have waited long in the hopes of 

 being told what constitutes a ' true schist,' but our patience has 

 not yet met with the reward it merits. The author is of opinion 

 that ' the alleged distinction between schist-like rocks and schists 

 of pre-Cambrian age requires great delicacy of definition.' This is 

 delicately put, and will command the assent of most geologists " 

 (Nature, vol. xliv, p. 26). 



In the following paper an attempt has been made to ascertain 

 the different meanings attached by authors of repute to the terms 

 Schist, Slate, Phyllade, and Phyllite. All the petrologists quoted 

 are cited as authorities of equal value, and as evidence of the 

 meanings actually current as to different technical terms at the time 

 of writing. No attempt is made to show that one is more correct 

 than another, nor is there any suggestion of such a thing. 

 Webster's Dictionary is quoted, not as an authority on technical 

 terms, but as the glossary of easiest access to the ordinary reader. 

 Webster's definitions may be taken to be the interpretation of 

 scientific terms as popularly accepted. 



Schist, Slate, Phyllite, Foliation. — The confusion that has arisen 

 in the use of the foregoing terms is due to the same words being 

 used by geologists in two or more different senses. 



Schist and slate are etymologically identical, absolutely synony- 

 mous and interchangeable. Both terms signify rocks that split. 

 De la Beche commonly used the word slate, to the entire ex- 

 clusion of schist. He tells us how a deposit of quartz, felspar, 

 and mica would become gneiss ; " while one formed only of grains 

 of quartz and mica would become mica slate " (Geol. Obs., p. 705). 



1 Mr. C. Reid mentions that in some of the Coral islands of the South Seas, 

 boulders which reach the shore on drift-wood are the perquisite of the chiefs. 



