SYS ISR AING, SIRI OT AAI a (VG 351 
with the aid of the exterior characters. His divisions are styled 
classes, orders, genera, and species. There are but two classes, 
z,; (1) “ Roches homogenes ou simples; (2) “Roches hété- 
rogénes ou composées.”’ In this respect Brongniart is more logi- 
cal than von Leonhard. The Homogeneous rocks are defined as 
those, that appear, 70) be ‘soto the naked ‘eye. Dhere are) two 
orders under the class of Homogeneous rocks, namely: (1) 
‘Roches phanérogénes (of distinct known mineral species); (2) 
‘Roches adélogénes” (of unrecognizable constitution). This 
division is logical enough, but it is based on a criterion of pri- 
mary value which is clearly applicable to a// rocks. The result 
is that under Homogeneous rocks are included, in the second 
order, a number of substances, including clays, dense schists, 
trap, basalt, and other igneous rocks, of really heterogeneous 
character. 
The Heterogeneous rocks are also divided into two orders : 
(1) ‘Les roches de cristallisation; (2) Les roches d’agréga- 
tion.” This distinction really introduces the factor of condi- 
tions of origin, which fact Brongniart seemingly did not 
recognize, as he makes no comment upon it. 
When it comes to finer subdivision we find that this system 
is almost purely mineralogical, as its title claims. Structure is 
relegated to a very subordinate role, wherever it is convenient to 
apply it. 
Systems of von Leonhard and Brongniart comparea.— lf we now 
review the construction of the systems proposed by von Leon- 
hard and Brongniart, which may be taken as the real starting 
point of systematic petrography, we find that both resorted at 
once to an expedient which expressed the fact that in their day 
the composition of many rocks was unknown. And the princi- 
pal inconsistencies of the two arrangements came from their 
treatment of the aphanitic rocks of all kinds throughout the 
schemes. But we must admit that these substances could not be 
correctly classified by composition so long as their character was 
so nearly unknown. 
Both these masters relied mainly on sneral composition, but 
