SILLIMAN'S INTRODUCTION. 7 



is not perfect coal ; it burns with a sharp and acrid odour like the 

 smoke of a wood fire, and is in fact wood only partially altered by 

 inhumation. It is most abundant in the more recent geological 

 formations, especially in tertiary sands, gravel, and clays, in which 

 true coal has rarely been found. The trees that are buried in the 

 recent alluvial and diluvial deposits are little altered, and are, for the 

 greater part, referable to existing floras ; sometimes they are 

 flattened by pressure, and altered in their texture, and even partially 

 carbonized. 



Limestone and Marble. — Limestone, including chalk and marble, 

 is a most useful substance. The ancient Grecian temples give 

 decisive proof of the durability of marble ; and its beauty, even after 

 the lapse of two or three thousand years, and after innumerable 

 aggressions by the violence of war, and the depredations of anti- 

 quaries, more destructive than the action of the elements, is in 

 many instances not entirely destroyed. Limestone is the most 

 important ingredient in mortar, and of great value in soils. But it 

 is not necessary to enlarge on a subject so generally understood. It 

 is sufficient to remark, that in the selection of limestone for archi- 

 tectural purposes, or as a fertilizing ingredient for soils, geological 

 skill will often prove of great value ; and in cases of high responsi- 

 bility, such as those of public edifices, intended to endure to distant 

 ages, the united services of the geologist, the stone-mason, and even 

 the quarry-man, may well be put in requisition, for practical artists 

 often acquire the skill to judge very correctly of the value of 

 materials. The experience of all antiquity proves this to be 

 true. 



Various Rocks. — The same view may be taken of granite and 

 sienite, and the slaty rocks of that family, of porphyries, traps, and 

 soapstones, of sandstones and puddingstones, or breccias. All 

 these are employed either in constructing the external walls, or 

 in forming the interior decorations of buildings, as well as in forts, 

 docks, bridges, quays, aqueducts, and roads ; and it is of the utmost 

 importance that time and expense should not be bestowed upon 

 materials that are faulty, or worthless ; for some kinds of sandstones, 

 limestones, and even granites, crumble away, upon exposure to the air 

 and the weather, and thus produce deformity and dilapidation, where 

 everything should be solid and enduring. Li general, this error may 

 be avoided by a careful observation of the effects of time and the 

 weather upon such masses of rocks as chance to be prominent above 



