506 Prof. Bonney—Crystalline Schists of the Alpine Chain. 
of rock specimens, which fills about thirty drawers, and of about 500 
slices for the microscope.’ 
Thus, as my opinions have not been formed without the expenditure 
of time and trouble, I hope to be forgiven for expressing them with 
some confidence. Experience had already taught me, before beginning 
this work, that even accepted authorities might prove to be misleading, 
so I consulted, as is my usual practice, the ‘‘ literature of the subject” 
only so far as to ascertain the localities where the most conclusive 
evidence on the points at issue was likely to be found. For the sake 
of brevity I shall restrict myself to two questions, which seem to me 
the most important of those raised by Dr. Preller: (1) the origin of 
the pietre verdi, including their relation to the associated rocks, 
and (2) whether certain Alpine gneisses and crystalline schists are 
rightly regarded as Palseozoic or early Mesozoic in age. 
The pietre verdi, according to the classification adopted by 
Dr. Preller (see p. 160 of this’volume), comprise, in addition to the 
hornblendic and chloritic schists which the name commonly covers, 
dolerites, diabases, and gabbros with all their varieties, lherzolites 
and serpentines, and even porphyrites. This classification, in my 
opinion, is open to the fundamental objection that it includes, with 
the pietre verdi (a series of rocks which, whatever may be their 
origin, have much in common), a number of others which, though to 
some extent related to them in mineral and chemical composition, 
can often be proved to be intrusions of later date. Probably this is 
always the case, but in a highly folded region we cannot obtain such 
clear evidence of it as in one which is comparatively undisturbed. 
Still, in many cases, we cannot doubt the intrusive character of 
certain dioritic rocks, and of many masses of gabbro and serpentine 
(altered peridotite), and we lose rather than gain in clearness of view 
by including these among the pietre verdi, instead of separating them 
as is done on most of the large-scale Alpine maps, such as those of 
the Swiss Geological Survey. The green schists—griiner schiefer or 
_schistes vertes—of these maps, wherever I have seen them from the 
Viso to the Glockner, have much in common. In the field they 
generally exhibit a more or less ‘slabby’ aspect, and a fine-grained, 
schistose structure, being often well foliated and sometimes showing 
mineral banding. The microscope proves them to consist (in variable 
proportions) of hornblende (sometimes glaucophane), chlorites, felspars 
(a secondary albite often being rather conspicuous), iron-oxides, and 
perhaps quartz. They are not seldom distinctly cut? by masses 
of serpentine (altered peridotites) and gabbros. The latter are 
occasionally very fresh, but more often the felspar is replaced by 
‘saussurite’ and the augite or diallage by hornblende—the well- 
known smaragdite-euphotide*® of the Saas-thal being the most 
remarkable variety. For some years I inclined to regard the griiner 
schiefer proper as being originally volcanic rocks—basaltic tuffs and 
' Both these collections have been given to the University of Cambridge, and 
are in the Sedgwick Museum. 
* As, of course, certain apparently older gneisses and schists may be. 
* As this term, proposed by Delesse, has for long been familiar to English 
geologists, I do not see why Dr. Preller should call it euphodite (p. 161). 
