Til E 
309 
IV. AL : Tii ! 1RANEAN IN AT 1 UAL! ST 
3. Even among this species a vast number occur 
which live in milder climates and ..voder entirely 
different eomtid ms and influence'-, and we, find 
some families repn sent .-d by only oo forms, lack- 
ing entirely oriviaul Arctic species. 
4. Tire differences between the faunas of the 
Eastern and Western Hemispher; s are slight, and 
generally sneaking, those f inns which are most 
frequently represented in one are found in the 
larger proportion in the other. 
The Relationship of the Structure ol 
Rocks to the conditions of their 
Formation. 
By H. J. Johnston La\ is, m.d.,f.g.s.,etc. 
Amphiboly. — This mineral lias baffled all the 
attempts of the chemist to prepare it artificially 
otherwise than as a sublimation. AVhenever its 
elements were fus id y, or a com j fu- 
ture, was fused, the only product was its ally — 
pyroxene. Our entire acquaintance with arnphibole 
indicates it as a mineral crystallized under pres- 
sure, and probably from an acquiferous magma. 
Its continual occurrence in syenites and allied 
rocks show it to be easily erystallizabie under the 
conditions which the . rocks came into existence. 
I have found it in basic pumi v -. continually ac- 
companying orthoclase (of which we shall speak 
next) in the more vitreous and early stage of the 
explosive eruption ejectamenta of Vesuvius, lloc- 
cainonfina, &c. In the later stage • of the same 
eruption it does not appear to have increased in 
size or abundance, whilst it is often enveloped in 
pyroxene; and this latter species is spread throu gh- 
out the mass, increasing in pro;. or;, ion as the 
rock approaches the lava type. This is the more 
remarkable, because we know that arnphibole is 
more fusible than pyroxene; whereas, if we exclude 
change of pressure, Ac., it should have crystallized 
later. This fact alone is quite sulii.-icnt to dispr »ve 
any relationship between the. fj wing-point of a 
mineral and its order of crystallization. Where 
amphiboly L found in a lava, we have evidence 
that it existed as such before the eruption of that 
material. It is not at all an uncommon mineral 
lining vesicular cavities; but it there shows itself 
to have been deposited by sublimation, which is 
borne out by its discovery under similar conditions 
in some furnace scorias. (1) 
Orthoclcistic felspar was obtained by M. Stanislas 
Mounter (2) by fusion and subsequent recuit of 
acid rocks. The product, however, only consisted 
of crystalline concretions, having the composition 
of uthoclase. M ieroliths only rewarded the efforts 
of Messrs. Fouquei and M. Levy (3) after a long 
recuit of eight clays. These facts are thoroughly 
borne out by the basic pumices. Those that were 
cooled very rapidly in the first eruptive stage 
exhibit large, well-formed crystals of sanidine as- 
sociated with arnphibole, showing the similar con- 
d : i ions under which the two minerals were formed. 
(4) In the latter stage of these eruptions the large 
. ! ratals have not increased in number or size (?); 
but from lire sioower cooling a few microliths have 
formed. Another proof is to be found in the 
o. ourrenci of fragments of a porphyritic rock, which 
is only the pumice magma that, in some outlying 
fissure, has cooled under pressure, and in some 
ca undergone secondary alteration. This shows 
the sanidine crystals still larger and more perfect. 
This i..»ek may be traced by gradations to a syenite 
-like rod:, in which the amorphous magma is enti- 
r d y converted into formed matter. In the basic 
1 o. as, which are identical in composition with the 
a 1 eve pumices, the sanidine only occurs as very 
small crystals or microliths, as the magma rising 
quickly to the surface has little time to partially 
crystallize under pressure. On the contrary, after 
xtrusion, the lava will cool very much slower 
than the pumice, so that the prolonged recuit will 
br highly favourable to development of orthoclase 
T . .Mho a.,d even small crystals. These facts 
are well borne out by Vesuvius in its pumices and 
modern l iras, whilst the outflows Phase IV., 
following immediately on a pumieeous phase, hold 
an intermediate place with iward to their mono- 
elinic. felspars. It is not an uncommon thing in 
(1) M.L. Bourgeois, Op.cit., p. 110. 
Comptes rendu y 1880, t. .re., p. 1009. 
(■■) Comptes rendus, 1878, t. Ixxxvii., p. 7 00. 
( •) Whether these are real.li/ orthoelastic is gen~ 
era llg a vtri/ diffimlt matter to determine 
