139. Marbles may be divided up into two great divisions, and even these are somewhat interchangeable, and it will depend upon the angle that the slabs have been sawed as to whether they should be placed in one or the other division. These two divisions are the striated marbles, which includes the veined and serpentine marbles, and the conglomerate, which includes most of the onyxes and marbles which show but little veining. As stated before this arbitrary division is sometimes misleading, as a striated marble will show up in chunky formation if sawed in a certain way, and again considerable veining will show in a conglomerate if sawed at the right angle. The placing them in one or the other of the two divisions applies only when they are sawed in the natural and customary way—squarely through the block where they show their character more fully. In some varieties, however, as in some of the onyxes, there is little regularity of formation and serpentine forms will be found bordered with large chunks of conglomerates, and the latter with more or less of well defined serpentine formation intermingled with it. This really is what makes the chief beauty of the onyxes. The students should study the forms taken by marbles thoroughly as possible in order to store up in their memory the innumerable variations of forms of all the several sorts of marbles which they come across.
QUESTIONS ON LESSON XXVI.
135. What is said relative to the imitation of marbles by ancient civilizations?
136. What uses of marble imitations are made of today?
137. To what parts of interior decoration are they best adapted to?
138. What is said of the irregularity of form in marbles?
139. In how many divisions can marbles be placed in?
LESSON XXVII.
GENERAL DIRECTIONS FOR MARBLING.
140. Under the name of marble are included all variegated stones, and therefore the term “marbling,” like that of “gilding,” which is applied to gold, silver and all metal leaves means considerably more than it should. Truthfully speaking marbles are of lime formation, while granites and jaspers and many others are not. The coloring of the veinings are due to various causes; various substances being deposited and enveloped in the lime formation, some by infiltration of metallic ore beds above the lime formation, which found their way through some of the softer fissures of the imperfectly formed limestone and deposited there. Again, as in honeycomb limestone, substances which deposited along with the lime, but which being more soluble have been washed out of it, leaving the lime full of small holes, as the name indicates. These being subsequently filled up with colored deposits.
141. General directions will be given in this and subsequent lessons as to “how” the various operations used for imitating marbles are performed, and the student will be referred to them when the special description of the various well-known marbles and other stones are given. The same operations being used in nearly all of them, being varied to suit the peculiar needs of each as required.