G Classification of Jloclis 
neighbourhood of igneous rocks moreover, or where great 
alteration lias taken place, it is often impossible to dis¬ 
tinguish in hard specimens, and without the aid of sec¬ 
tions, between the truly igneous rocks and the metamor- 
pliic. So far the principles on which rocks are named 
and classified are very simple; but when we come to the 
aqueous rocks a complication takes place, which often 
throws great difficulties in the way of the student. In 
the first place the aqueous rocks, like the igneous, derive 
their names, considered merely as rocks, from their 
mineral character, such as sand, sandstone, gritstone, 
conglomerate, clay, shale, slate, chalk, limestone, marble, 
gypsum, or coal. Inasmuch, however, as these rocks 
form a great series in which their several varieties occur 
over and over again, alternating one with the other in 
every possible way, we must, when we wish to speak of 
any portion of that series, use terms differently derived. 
We must have terms to express which mass of sandstone, 
clay, or limestone we mean to speak of. It is indifferent 
what these terms are : generally speaking, the less 
meaning they have the better. Descriptive names arc 
often bad, as they mostly describe mere local accidents ; 
and we have to speak of black limestones as cretaceous 
rocks, of sandstones and shales as oolitic, or of slates 
containing no coal as carboniferous rocks. The practice 
most prevalent now, and for the present the best, is to 
name the rocks from the district where they are best 
exhibited, or where they were first described, as, Silurian 
or Devonian. 
In classifying the series of aqueous rocks of any 
country, regard must be had to three characters :—1st, 
mineral structure;—2nd, organic remains ;—3rd, order 
of superposition. The mineral characters of aqueous 
rocks are the most obvious, but, as far as classification 
goes, the least valuable. It is, however, absolutely 
