Lithology for Museum Purposes. — Gratacap. 287 
world, whereby, especially in igneous rocks, variations and 
contrasts can be instructively indicated, as for example the hy- 
persthene belt of eruptives along the Rocky mountains, and 
the more generally augitic and hornblendic series in the east, 
the consolidated limestones of the ^lississippi basin, as con- 
trasted with the crystalline marbles of the Appalachians, and 
the association of the crystalline schists with axes of elevation 
and folding, overthrust and contraction. Distribution could 
be more rigorously treated in an exhibition of bathysmic as 
well as superficial extension, the older lower igneous rocks 
usually presenting acid features as succeeded by more basic 
overlying lava flows. Again local peculiarities admit of much 
interesting treatment as the foyayte dyke in New Jersey, and 
the rare occurrence of phonolytes in the United States. For 
popular use, however, broad, easily grasped features are to be 
preferred. 
The lesson is more easily learned, is generally more forci- 
ble, and is better remembered. 
VII. Life History. 
The life history of a rock can be made difficult or simple. 
Its embryology, so to speak, can be introduced, its phylogeny 
and its disintegration. We can point out the constituents of 
a rock, show from what sort of a general mass, magma or sed- 
iment these constituents have originated, illustrate phases in 
its construction and then pass from its typical forrn to those 
changes which, whether by uralitization or decomposition, 
mark its lateror last stages. More usuall}' such a stud}' of its 
life history would mean, with a rock, a portrayal of its dissolu- 
tion, and rock' weathering could be well illustrated by a direct 
transference to the cabinet, of the words and examples of Prof. 
Merrill's book. 
VIII. Geological Occurrence. 
This section practically is a selection of geological zones, 
depicted by representative rocks amongst which fossilifcrous 
rocks would be included. Such zones are preferably to be 
made broad ones, as Archaean, Laurentian (Huronian), Al- 
gonkian, Silurian, Devonian, Carboniferous, Jura Trias, Cre- 
taceous, Tertiary, Recent. Two series, one from Europe and 
one from America, could be profitably compared. 
