142 Correspondence—Mr. S. Allport. 
of succession of the strata, a rectification of the classification of the 
fluvio-marine series is rendered necessary, and it is proposed to 
divide them as follows :— 
1. The Hempstead Series (marine and estuarine), 100 feet. 
2. The Bembridge Group (estuarine), 300 feet. 
3. The Brockenhurst Series (marine), 25 to 100 feet. 
4. The Headon Group (estuarine), 400 feet. 
By this new grouping the strata of the Hampshire Basin are 
brought into exact correlation with those of France, Belgium, North 
Germany, and Switzerland ; and the whole series of fluvio-marine 
beds in the Isle of Wight, which are shown to have a thickness of 
between 800 and 900 feet, are proved to be the representatives of 
the Lower and Middle Oligocene of those countries. The use of the 
term Oligocene in this country is advocated on the ground that by 
its adoption only can we avoid the inconvenient course of dividing 
the fluvio-marine series between the Eocene and the Miocene. 
COR Ria S 2 @ AND mara Saber 
fe EE 
PETROLOGICAL NOMENCLATURE. 
Sir,—In a paper on the Rocks of Brazil Wood (Gro. Mac. 1879, 
Decade II. Vol. VI. p. 481), I described the so-called peculiar gneiss, and 
considering that it had been wrongly named, I called it a micaceous 
schist. As I strongly object to changes of names without sufficient 
reason, I wish to add a few words on the subject, more especially as 
it appears from a letter (see Grou. Mac. Jan. 1880, p. 47) that Prof. 
Bonney is not persuaded that I have improved matters by the change 
just mentioned. After carefully reviewing what I said on the sub- 
ject, I venture to think that I rather understated my case, and that 
the objections to the old name are really fatal to its retention. The 
fact is, that the rock in question has neither the chemical composi- 
tion, mineral constitution, internal structure, nor even the external 
appearance of gneiss—a rather formidable combination of objections. 
In the first place, a reference to Roth’s tables of analyses will show 
that the amount of silica in micaceous gneiss is, on an average of 
sixty analyses, quite 70:00, whereas the quantity in the Charnwood 
rock is only 54-01. 2nd. There is a total absence of felspar from 
the mass of the rock, although there is a little in the narrow band at 
the actual junction, but even this may belong to the granite. There 
is also far less quartz than I have seen in any true gneiss—merely a 
few grains being scattered here and there through the mass. As 
regards structure, I said there was no foliation, but that the rock 
was rather fissile in one direction. I here used the term foliation 
in the sense attached to it by Darwin, and which I think it ought 
always to retain, namely, a separation into folia or layers of different 
mineral composition. It will of course be admitted that a rock may 
be more or less fissile, or schistose, without being either a slate, or a 
foliated schist ; in fact, among the older rocks, and in some districts, 
such rocks are of common occurrence, whereas foliated rocks are 
