140 
REVIEWS. 
tive theory of igneous rocks is given briefly, and is worthy of attention, 
as it appears to he gaining ground among Continental thinkers. On the 
delicate subject of Metamorphism Mr. Jukes’s views are sound, and of 
the English school ; and he attributes the principal share of metamor- 
phic action to the transforming agency of the igneous rocks, illustrating 
this action from the well-known examples of the slate rocks of Wicklow 
and Wexford:— 
“While we give full allowance to the importance and magnitude of the metamorphic 
effects produced by water at whatever temperature, there are yet still greater and more 
general changes which we must believe can only have been effected by the action of heat, 
too great to allow of the presence of water. 
“ When we see whole mountain ranges, and whole districts of country, consisting of 
rocks that have more or less analogy in structure and constitution to rocks known to be 
of igneous origin, we cannot help feeling convinced that igneous action must in some way 
have been concerned in their production. 
“ When we find that these rocks have every gradation, from such as might have 
been once molten, into rock which we know to have been mechanically deposited under 
water, we are compelled to conclude with Lyell that these rocks are altered or meta¬ 
morphosed by heat from their original aqueous and mechanical formation into a state 
more or less nearly approaching true igneous rocks. 
“ Our belief in the truth of this metamorphism becomes certainty when we see these 
rocks always occurring on the flanks of masses of granite, and examine a district (such 
as Wicklow and Wexford) where both large and small masses of granite appear, and 
find these metamorphic rocks, not only always accompanying the granite, but occurring 
nowhere else except in the neighbourhood of granite or granitic rocks, and their extent 
always proportioned to the size and extent of the particular granite mass they mantle round. 
“ It is by no means intended to assert that the neighbourhood of granite or igneous rock 
is the only source of heat from which this metamorphosis can arise. Should any mass 
of rock, capable of alteration, be so deeply buried in the earth as to be brought within 
the reach of any centre of heat whatever, the same effect would result; and it is quite 
possible that a far greater intensity and wider range of heat may be thus reached than 
could proceed from the mere intrusion of a more or less isolated mass of igneous matter 
into spaces which were naturally of a lower temperature. But as an intrusive mass of 
granite must be a source of great heat, and as the metamorphic effects in question are 
found always to accompany it, we are obliged to look upon heat as the cause of the effect. 
“ This effect of intense heat may doubtless be variously modified by the previous pre¬ 
sence or absence of water, and by the various mixtures of mineral matters occurring in 
the different rocks before alteration.” 
Mr. Jukes should not, holding, as he does, such correct views on the 
subject of metamorphism, have elsewhere quoted Bischof’s wild specu¬ 
lations on the same subject, without some caution to put the student on 
his guard against his theory, which has been well described by saying, 
that, according to Bischof, the end of all things is mica. 
In the Petralogical portion of the Manual Mr. Jukes is quite at home, 
and we know of no text-book in which so much useful and practical 
information on this branch of geology is given. It is the most original 
portion of the work, and is the only part of it that is illustrated. 
The woodcuts themselves are well intended, but badly executed, and 
coarse, and occasionally printed upside down. The author has good 
cause to complain of the printers and readers who revised his sheets. 
We are glad to find that Mr. Jukes, who, from his large experience 
as a geological surveyor, is well qualified to judge of such a question, 
