Correspondence—Ur. Bernard Hobson. 91 
GRANITE. 
Sir,—I can only express amazement that such a paper as that of 
Mr. T. R. Struthers on Granite (Guon. Maa. 1892, p. 561-4) should 
have been written in the year of grace 1892. 
Mr. Struthers begins by attributing to geologists the absurd theory 
that the supposed granitic foundation of the Earth’s crust was 
formed after the overlying crust itself. A perusal of pages 86 to 88 
of Rosenbasch’s paper “ Zur Auffassung des Grundgebirges””' would 
have shown the groundless nature of this statement. 
Then follows a delicious example of a non sequitur in the observa- 
tion “that erupted rocks, whether volcanic or trappean,” have 
apparently been derived from granite, for in common with them 
it consists mainly of silica, alumina, potash, soda, lime, magnesia, 
and iron.” Why granite, to the exclusion of syenite, diorite, gabbro, 
peridotite, etc., is to arrogate to itself the honour of being the mother 
of all erupted rocks is not even supposed to require explanation. 
Then we have a quotation of a most unfortunate remark of Beete 
Jukes (no page or reference given; I find it in the 2nd edition of 
his Manual of Geology, 1862, omitted apparently in the 3rd edition, 
1872), from which Mr. Struthers evidently concludes that any lava 
stream, of no matter what chemical composition, could be traced 
within the earth to granite. Let the petrologist imagine tracing 
a limburgite lava stream to a granite magma. 
Mr. Struthers states that ‘the hydrothermal conditions under 
which granite was formed . . . were peculiar to a particular period 
of the world’s history, when a sea of high temperature overspread 
its entire surface before any dry land had appeared.” Anything 
more contrary to known facts than this statement can hardly be 
imagined. The Dartmoor granite and the Brocken granite alter 
rocks of the Carboniferous period. Does Mr. Struthers suppose that 
no dry land had appeared at that period or that the fish of those 
days swam about in boiling water ? 
He has no hesitation in saying (apparently only from the examina- 
tion of Lyell’s figure (Hlements, 1874, p, 552) that the granite of 
Sharp Tor, Cornwall “is a fine example of bedded eramilic originally 
discharged in successive submarine sheets.” 
Unfortunately Lyell does not state the scale of his drawing, other- 
wise Mr. Struthers would probably see the untenable nature of his 
interpretation of the well known mural weathering of granite. An 
examination of the rock of the apparent pile in question would pro- 
bably show under the microscope complete continuity in the un- 
weathered portions between the seemingly separate lenticles. If 
Mr. Struther’s interpretation of mural weathering were correct the 
lenticles should be of immense size, whereas every one practically 
acquainted with the subject knows that they thin out or are discon- 
tinuous usually in a distance of a few feet. Bernarp Hopson. 
Owens Cottecr, Mancuerster, Dec. 19¢h, 1892. 
1 Neues Jahrbuch fiir Mineral, 1889, Bd. ii. 
2 Mr. Struthers still classifies rocks into, among other groups, ‘‘ trappean’’ and 
actually calls syenite a trappean rock. 
