Tie Mediterranean naturalist. 
a fl!>ontbl\> IRevtew of Natural Science, 
Yol. II., No. 19. MALTA, DECEMBER 1st. 1892. 
/ 4/- Per annum 
\ By post 5/- 
CONTENTS. 
— «.«■ — Pag. 
1 The Relationship of the Structure pf Rocks — H. J. 
Johnston La vis; M.D., F.G.S., etc. 
2 Birds of Malta 
3 The late eruption of Pantelleria 
i Entomological Notes— A. Caruana Gatto, LL.D. 
283 
285 
286 
287 
5 Science Gossip:— A peculiar grass— Potato disease— 
Curious insects -The glaciers of the Alps— Electri- 
fied water—' The English race— Life forms Austra- 
lia— The Indian twins— Ancient method of <\ uarry- 
ing— A store house of energy— Insect life-Maltese 
Echinoidea— Effect of temperate regions on Civili- 
zation-Cultivation of marine life — Wanted a bio- 
logical station for Malta, etc. etc. 287 
6 The Canary Islands 291 
7 Anew VineDiseasc -A. Caruana Gatto, LL.D. 291 
8 Comino and its islets 292 
9 The Italian Geographical Congress of 1892 295 
lb Botanical Notes — Major Godfery, A.S.C. 296 
11 Notes and News : — Maltese tomatoes — Earthquake 
in Sicily — Manufacture of blood-oranges— Medical 
Congress — Malta Vine disease— Malta potato di- 
sease— The Mosquito — Maltese fisheries — Marl 
Beds of Malta — Biological station at Montpellier — 
“Bird-Murder” in Malta, etc. etc. 297 
12 Coi'respondence :—" Birds v. Insects" 298 
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19 
The Relationship of the Structure of 
Rocks to the conditions of their 
Formation. 
By H. J. Johnston La vis, m.d.,f.g.s., etc. 
Let us now turn our attention to the mineral 
composition of an igneous rock. Any given magma 
will produce rock of the most varied character, 
according to the conditions under which consolida- 
tion takes place. Thus, for instance., a given dyke 
of magma might be a granite near its origin, high- 
er up its sides or whole may be pitchstone, and 
its centre a liparite or porphyry, whilst at the sur- 
face it would present itself as vitreous pumice, an 
ash, an obsidian or a quartz trachyte. It has been 
observed that granite veins branch out, and the 
ramifications may assume the type of felsite, which 
is of course dependent on the more rapid cooling 
just as in the case of the salband. Again, we have 
a series of gradations from a true leucitic basalt, 
such as the recent lavas of Vesuvius to a sanidine 
porphyry, to a highly crystalline syenite containing 
leucite, but more commonly nepbeline in the rocks, 
composing the ejected blocks. The generalization 
based upon the geological ages being characterized 
by different types of rocks is false, and is no doubt 
due to the depth to which denudation has extended 
It is a general fact that the slower the cooling takes 
place the more perfect will the crystallization be. 
This we have already spoken of when treating of 
the difference in the amount of gaseous constituents 
in a magma which, by bringing about great 
rapidity of cooling in explosive eruptions, makes 
the products tend to an amorphous rather than a 
crystalline condition. One remarkable fact well 
borne out by the lavas derived from the different 
eruptions of Vesuvius is that the size of the crys- 
tals are much greater in the little oozing forth of a 
small quantity of lava from the crater than in the 
great outpour. This will be evident, as in the first 
case the lava has been, in a 3tate of simmering in 
