366 CENTRAL MINERAL REGION OF TEXAS. 



speckled and mottled varieties may be obtained. These will frequently polish 

 with good effect, and some outcrops show by their style of weathering that 

 certain oxidation tints of value for ornamentation may be utilized. In work- 

 ing this class, however, great care will be necessary to select only those quali- 

 ties in which the mica is well distributed and in comparatively fine scales. 

 Otherwise serious blemishes may result from irregularity of "weathering." 

 For paving blocks, foundation stones, and special uses, much of this mate- 

 rial may be utilized eventually at little cost. 



B. THE COMPRESSED GRANITES. 



There are many of the Archaean granites which appear as if tightly wedged 

 in between the adjoining strata, although the result is perhaps more often 

 due to the expansion produced by the crystallization of the magma. For 

 this last reason the material is frequently an aggregation of large interlock- 

 ing crystals, without coherence enough to render the rock useful for any pur- 

 pose where stress is anticipated. Occasionally some very fine crystals of 

 orthoclase and microcline of immense size, or tough masses of graphic granite 

 admitting of a high polish, occur in similar situations. Adularia in crystals 

 of good size is also abundant, and this may find a use economically, if not 

 for building directly. 



C THE BLOCK GRANITES. 



This name is proposed for a class of granites which occur in various parts 

 of the Central Mineral Region, and which may represent more than one up- 

 lift. They are not yet fully understood, and there is here no intention of 

 putting them together except for the convenience of treatment. They shade 

 off into dimension granites upon the one hand and into fissile granites upon 

 the other, in a general sense, and yet they are distinctive, more by reason of 

 their texture than their structure. At the same time it is possible that some 

 of the friable granites, or "rotten granites," may be only the block granites 

 undergoing a process of decay. But a commercial classification must neces- 

 sarily deal with present conditions, and upon that principle this class name is 

 justifiable. I would include under it all those granites which are solid 

 enough to withstand sharp blows from a single-hand hammer without shat- 

 tering, except along the line of impact, so that they may be readily broken 

 into blocks. But, in addition, they must have definite joint or lamination 

 planes, usually induced by the mode of arrangement of the large crystals of 

 feldspar. Any of the other types, howsoever well they may be broken into 

 shape by careful trimming, do not at once assume the block form character- 

 istic of this class. Thus restricted, there is considerable variation in the 

 solidity of the blocks, even when they are not undergoing rapid disintegra- 

 tion. This difference arises from the irregularity of coherence among the 



