474 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



76,835, U. S. N. M.), still retaining the granular character and 

 much of the original structure of the sandstone, but crushing readily 

 between the thumb and fingers. Under the microscope this type 

 shows interesting structural changes which are only in part brought 

 out by the photomicrograph reproduced in figure 2, plate lxxi. A 

 portion of the quartz granules retain their original characteristics. 

 A larger portion are crushed and more or less distorted, though re- 

 taining their limpidity and high polarization colors. In many in- 

 stances two adjacent granules are crushed and fractured at point 

 of contact, as though they had been struck a sharp blow with a 

 Tiammer. This crushing has at times been carried so far that the 

 rock is reduced to a fine sand or flour (Cat. No. 76,840, U. S. N. M.), 

 each particle of which is as sharply angular as though disintegrated 

 "by a blast of dynamite (see fig. 126). Of greater significance from 

 the present standpoint is the presence in the still firm rock of a large 

 number of granules which are so completely changed as to give 

 rise to forms at first glance scarcely recognizable as quartzes at 

 all. A description of these is given in the discussion of the next 

 or second phase of the metamorphism. 



In this second and very complete phase the original granular 

 structure of the sandstone has almost wholly disappeared. The 

 rock (Cat. No. 76,837, U. S. N. M.; fig. 1, pi. lxxii) is chalk white 

 to cream yellow in color, quite hard, though in thin fragments 

 readily broken between the thumb and fingers, and lacks entirely the 

 arenaceous structure. It resembles the decomposed chert quarried 

 at Seneca, Missouri, under the name of tripoli, more than any other 

 rock that the writer can call to mind, although on casual inspection 

 it might readily pass for an old siliceous or calcareous sinter. This 

 material, Mr. Tilghman writes, occurs sporadically throughout the 

 pulverulent material, of which it constitutes some 2 per cent in bulk, 

 and in fragments from the fraction of an inch to 10 or 12 feet in 

 diameter. In one instance the drill passed through a body of it 

 some 50 feet in thickness at a depth of 500 feet below the surface. 

 In the mass this variety shows an uneven platy structure extending 

 across the original, almost obliterated, lines of bedding. The general 

 structure as seen in thin-sections is shown in figure 3, plate lxxi. At 

 first glance such would be pronounced to be a holocrystalline rock. 

 It is in fact an aggregate of closely interlocking quartz granules with 

 low and very uniform relief, dull colors of polarization, and in the 

 majority of instances a marked rhombohedral cleavage. So striking 

 are these features that at first the true nature of the mineral was not 

 recognized. Extinctions are often undulatory, indicating a condition 



