THE ORIGIN OF IGNEOUS ROCKS. 115 
them greater fluidity. Since the source of the fused mag¬ 
mas is in heterogeneous material, and since the increase of 
temperature is an unexplainable condition, which may be¬ 
have very erratically, it is to be expected that the sequence 
of eruptions will not always be constant, and apparent ex¬ 
ceptions are to be looked for. In fact Dutton is surprised 
to find as much uniformity in the order of succession of 
volcanic rocks as is observed. 
Since 1880 the science of petrology has been developing 
along new lines, and its followers have been busy establish¬ 
ing in great detail the exact mineralogical character of all 
crystalline rocks, as well as advancing their chemical inves¬ 
tigation. A more thorough exploration of their geological 
occurrence and the finding of more favorable localities for 
the study of their field relationships have led to a clearer 
understanding of their true nature and have paved the way 
for an advance in the theory of their origin. Within recent 
years no comprehensive discussion of the nature and origin 
of igneous rocks has been published, but more or less frag¬ 
mentary contributions to the subject have appeared. 
In 1885 Teall * discusses the possibility of a partial crys¬ 
tallization and separation by gravity of an originally homo¬ 
geneous magma in explanation of the varieties of rocks in 
the Cheviot District, but adds nothing new to the theories 
already based on this idea. 
A new impulse was given to the science by the work of 
Lagorio f “ On the nature of glass base, as well as the processes 
of crystallization in eruptive magmas,” published in 1887. 
As the title indicates the paper is confined to a study of 
the glassy groundmass of rocks and to their crystallization. 
It treats the question from a chemical and physical stand- 
* Teall (J. J. H.) On some Quartz-felsites and Augite-granites from the 
Cheviot District. Geol. Mag. 8°. <London, December, 1885, vol. 2, No. 
3, pp. 106-121. 
f Lagorio (A.) “Ueber die Natur der Glasbasis, sowie der Krystallisa- 
tionsvogange im erupbiven Magma.” Min. u. petr. Mitth. 8°. Vienna, 
1887, vol. 8, pp. 421-529. 
