468 GEOLOGY OF NORTHWESTERN TEXAS. 



A great deal depends upon the mixing of these elements in proper propor 

 tion, and where that is not already done the material for this proper admix- 

 ture is nearly always convenient. Chemical tests of the clays will determine 

 very nearly what element would be lacking and how it could be supplied at 

 any given locality. 



In selecting clays for making bricks there are a few things to be consid- 

 ered. The material should be such as to give sufficient hardness to enable it 

 to withstand heavy pressure and to stand the weather. It should be such 

 that the brick will be of uniform size, to secure uniformity in the settling of 

 the building, and to add to its beauty. It should have some material that 

 would cause the whole mass to agglutinate and form a compact mass. When 

 these matters are properly looked into and provided for there is no reason 

 why good bricks may not be made at almost any locality in this part of the 

 State. 



FIRE CLAY. 



This term has been used in this Report to designate a peculiar clay under- 

 lying the beds of coal. , Fire clay for making the quality of brick known as 

 "fire brick," which have a due admixture of clay and silica with the silica in 

 excess, with no material that will act as flux when burning or when exposed 

 to great heat thereafter, have been found in other coal fields and called fire 

 clay, and in calling the clay "fire clay" in this district I have simply called 

 it so from its position and not from its chemical properties. 



potter's clay. 



The clays of the Coal Measures are excellent clays for making pottery, but 

 have not been utilized for that purpose at any locality so far as I know. 



MARBLE. 



At several localities my attention has been called to beds of limestone that 

 were supposed to be marble, but in no place has there marble been found. 

 Hard limestones capable of receiving a high polish occur in places, but it is 

 doubtful if any of them will be of any economic value. 



Two miles and a half east of Graham, east of Salt Creek, is a bed of this 

 marble. On first examination it appears to be sandstone on the outside, 

 caused by the harder grains of the stone having resisted the weathering, giv- 

 ing the outer surface the weathered appearance of sandstone. This bed is 

 very compact and has been slightly metamorphosed, yet not enough to de- 

 stroy the forms of the fossils, but sufficiently to make it very solid. It takes 

 an excellent polish, and could be used for articles of furniture that would not 

 require much handling. 



