548 PROCEEDINGS OF THE NATIONAL MUSEUM. voi,. xxxii. 



of well-rounded quartz granules with an occasional grain of a plagio- 

 clase feldspar, and a little dust-like material in the interstices, but the 

 amount of interstitial material of any kind is very small. The general 

 structure of the stone is shown in tig. 1 of Plate Lll. This type passes 

 into what may be called the first phase of the metamori)hism — an 

 almost chalk}^ white rock — (Cat. No. T6835 U.S.N.M.), still retaining 

 the granular character and much of the original structure of the sand- 

 stone, and crushing readily between the thumb and fingers. Under 

 the microscope this type shows interesting structural changes which 

 are only in part brought out b}^ the photomicrograph reproduced in fig. 

 2 of Plate LII. A portion of the quartz granules retain their origi- 

 nal characteristics. A larger portion are crushed and more or less dis- 

 torted, though retaining their limpidity and high polarization colors. 

 In many instances two adjacent granules are crushed and fractured at 

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

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

 is reduced to a fine sand (Cat. No. 76840 U.S.N.M.), each particle of 

 which is as sharpl}" angular as though disintegrated b}' a blast of d^^na- 

 mite (see fig. 4, Plate LII).'^' Of greater signiticance from the present 

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

 granules which are so completel}" 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 phase of the metamorphism. 

 In this second and very complete phase of metamorphism the origi- 

 nal granular structure of the sandstone has almost wholly disappeared, 

 as have also the original lines of bedding (Cat. No. 76837, U.S.N.M). 

 The rock 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 decom- 

 posed 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 cal- 

 careous sinter. This material, Mr. Tilghman writes, occurs spo- 

 radicall}' 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 mass 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 directly across the original, almost obliterated, 

 lines of bedding. Under the microscope this phase exhibits certain 

 features new to the writer, at least, and which are quite at variance 

 with our ideas of the stable character of quartz sand. The general 

 structure is shown in fig. 5 of Plate LII. A more highlj^ magnified 



"This is the material referred to as "SiUca" by Messrs. Barringer and Tilghman, 

 and of which there are said to be "millions of tons." 



