536 Dr. CO. Callaway—Rock-Metamorphism. 
a statement. Jf I am wrong in affirming that chlorite may be con- 
verted into biotite, it is very singular that several foreign observers 
of undoubted competence should have independently arrived at the 
same conclusion. 
If this mineral transformation is an observed fact, it may seem 
unnecessary to discuss theoretical objections to it; yet, since com- 
petent observers have so recently called it in question on the ground 
of these objections, it would be inexpedient to pass them over. Is 
there any theoretical reason why, under the conditions existing in 
the Malvern rocks, biotite should not be generated at the expense 
of chlorite? I venture to think there is none, and this I will en- 
deavour to show. 
It is rarely possible to determine the species of chlorite in the 
Malvern rocks, as the mineral usually occurs in irregular patches 
and strings. It is identified by its habit, its colour, its pleochroism, 
and its very low interference tints. The biotite is easily recognised, 
and does not require to be described in this place. For our present 
purpose, it will be sufficient to compare the average composition of 
the two minerals. The following percentages cannot be very far 
from the truth. ; 
Chlorite. Biotite. 
Silica 36 306 out 33 ie ek 40 
Alumina ate oe wie 16 20¢ 506 18 
Tron peroxide and protoxide 5 ee 390 16 
Magnesia... ass 300 34 200 366 18 
Potash ae a0 300 — 20¢ 506 8 
Water 20 soc 200 12 ane noc — 
100 100 
In the change from chlorite to biotite, we have to account for a 
gain of silica, iron oxide, and potash; and for a loss of magnesia and 
water. 
The liberation of silica is one of the most obvious features of the 
first stage of metamorphism, viz. decomposition. The crushed 
diorite, which is to become a mica-gneiss, is quite dirty with dis- 
seminated iron oxide, and is often freely veined with calcite. The 
elimination of lime and iron oxide would, of course, set free plenty 
of silica. 
The last paragraph tells us where the iron oxides come from. 
That they are in part used up in the generation of biotite can 
almost be demonstrated under the microscope. In some slides, 
grains of opaque matter lie in the midst of blotches of chlorite, 
and round each of these grains is a halo of biotite, which can be 
identified by its brown colour, its strong pleochroism, and its high 
interference tints. In other slides, the chlorite is traversed by 
cracks filled in with opaque brown granular matter, and each of 
these cracks is fringed on each side by biotite. This opaque matter 
must be some form of iron oxide, it is not material which, and one 
seems driven to conclude that the pleochroic halos are causally 
connected with the union of the constituents of chlorite with the 
iron oxide. 
