284 The American Geologist. May, i899 
In other words, I would group under terms definitions of 
the megascopic features of rocks. Many more could be intro- 
duced, but in a popular exposition terms, thinly separated by 
scientific differentiations, should be avoided. Inasmuch as 
from beginning to end the scheme of rock presentation in- 
volves practically definitions, under term.s those only pertain- 
ing to unaided ocular recognition are included. In the next 
step forward the intelligent visitor is led to consider in Physical 
Condition the microscopic structure of rocks. 
III. Pliysical Condition. 
Under physical conditions it seems desirable to gather a 
series of examples illustrating the microscopic featvires of 
rocks. • A thin section enlarged by a low power objective, with 
label and colored or uncolored drawing, will fully meet the 
requirements. Such features or phases of rock structure 
should be chosen as will, in igneous rocks, demonstrate the 
progressive changes of crystalline state from glassy through 
devitrification to holocrystalline structure with clean intelligi- 
ble illustrations of idio- and allotriomorphism. The lesson to 
be taught is the imminent tendency to crystallization in molten 
mineral masses, a tendency only checked by sudden cooling, 
while the examples shown can be so selected as to most simply 
show interference of crystals, ground mass and the develop- 
ment of essential and necessary minerals. Besides these basic 
conditions, perlitic, lithiophysic, spherulitic texture can be il- 
lustrated. In clastic (sedimentary chemical) rocks the instruc- 
tion conveyed presents the differing stages of aggregation of the 
cemented or semi-crystallized contents, as in limestone, the 
varying composition of sandstones, and the change of clay into 
argillyte. In metamorphic rocks, rock sections of gneiss and 
marbles, and roofing slates, quartzyte, etc., would reveal sim- 
ply their ultimate structure, the process involved in their cre- 
ation being considered in the next section. Each group [igne- 
ous, clastic (sedimentary) metamorphic] of examples should be 
preceded by a definition of the group. This, on account of 
similar or identical definitions, following in later sections of 
the display, involves repetition. But repetition is not to be 
shunned or feared. In disclosing to the popular mind the 
principles, facts and conceptions of lithology, repetition in a 
