mt 
Correspondence—Rev. Prof. T. G. Bonney. 47 
_ species from the Bradford Clay. One is an Antedon, the oldest 
known, with no special characters; the other is an Actinometra, with 
a centrodorsal essentially like those of species now living in shallow 
water in the Philippines and Malay Archipelago. The oldest known 
Comatula, an Actinometra from the Bath Oolite, has similar relations, 
CORRESPONDENCE. 
See 
PETROLOGICAL NOMENCLATURE. 
Sir,—Mr. §. Allport’s excellent paper on “The Rocks of Brazil 
Wood” (Vol. VI. p. 481) incidentally suggests a general question 
to which, at the present time, it may be worth while calling atten- 
tion. In this particular case, however, I am not persuaded that the 
name ‘“micaceous schist” is any better than “gneiss” for the Brazil 
Wood rock. In writing of it I left the name gneiss, which had 
already been applied to it, unchanged, because I was not able to 
suggest a better. The foliation is so slight, as Mr. Allport observes, 
that the name schist (which calls attention to that property) does 
not seem appropriate; and we are not unfamiliar with rocks, to 
which we at present extend the term gneiss, which exhibit that 
phenomenon very imperfectly. A more serious objection is, as Mr. 
Allport remarks, that there is generally no felspar visible. The 
microscope, however, shows a third constituent, and the analysis! 
suggests the presence of an aluminous mineral. In this, if we 
assume that all the magnesia is present in a magnesia-mica, and all 
the soda in a soda-mica, we find (using rough approximations 
founded on some of the analyses given in Dana’s Mineralogy) 
that to form these micas we require for the one about SiO,=4°6 
Al,O,=2'2 FeOQ=2, K,O=°'8; and for the other about Si0,—7:0 
Al,O,=4:2 FeO=:3. Thus we have still left unemployed about 
SiO, = 42-4 Al,O, = 15°47, K,O0 = 2°8; this (with the H,O) 
seems to indicate some zeolite, which may have resulted from the 
destruction of a felspar. The rock then, though not strictly speaking 
a gneiss at present, may have been a kind of gneiss; though, under 
the circumstances, it is also possible that the felspathic constituent 
passed at once from the state of mud to its present condition. 
This rock, then, is an example of a class of difficulties unpleasantly 
familiar to every petrologist. With an abundance of synonyms in 
some cases, our nomenclature is very defective in others. Thus, 
following the lead of the older geologists, it does not recognize clearly 
enough that the metamorphic rocks are not so much a third class 
independent of the other two? and of equal value, but that every rock, 
whether igneous or stratified, may have its metamorphic representa- 
1 Water = 4°23 CaO = 2°18 
Si02 = 54:01 MgO = 1°30 
ASO =) 21:87 K.0 = 38°66 
Fe,0, = 5°38 NasO -= _1:00 
FeO = 6:24 
MnO = 0°63 100°45 
8.G.=2°85 (Q.J.G.S. Vol. xxxiv. p. 224). 
2 Using for the occasion the old division into igneous and stratified. 
