300 THOMAS L. WATSON 



not present in the groundmass may occur, in some cases, in the nodules. 

 As would be expected, difference in color for different parts of the 

 spheroid would naturally follow from such a structural arrangement 

 of the differently colored minerals, such as feldspar and quartz with 

 one or more of the dark silicates. In the Carolina rock the sphe- 

 roids are very generally composed of the dark silicates, in many of 

 which the light-colored minerals, quartz and feldspar, entirely fail, 

 hence they are very basic in composition and are of the same uni- 

 formly dark greenish color throughout. The feldspar filling the 

 interspaces is often penetrated by large laths of the black lustrous 

 hornblende, which may either have contact with, or extend from, 

 the spheroid, or be entirely separated therefrom and inclosed wholly 

 by the feldspar. (See Fig. 2.) 



MICROSCOPICAL CHARACTER OF THE ROCK. 



Six thin sections were prepared from selected chips of the rock 

 for microscopical study. Five of the sections were cut from the 

 nodules and one from a representative fragment of the interstitial 

 filling or matrix. The character of the sections was such that only 

 slight evidence was afforded of the structure of the nodules micro- 

 scopically, but the radial arrangement of the minerals composing the 

 nodules about a common center is entirely clear in hand specimens 

 of the rock, as indicated in the megascopic description above and 

 shown in Fig. 2. 



Diallage, green hornblende, basic plagioclase, microcline, quartz, 

 titanite, muscovite, calcite, zoisite, magnetite, and an occasional zir- 

 con are the principal minerals of the rock. Essentially the same 

 minerals are observed, as a rule, in both the matrix and the nodules, 

 but in different proportions; the matrix being composed very largely 

 of feldspar, with very subordinate amount of most of the other min- 

 erals, and the. nodules of the ferromagnesian minerals, with in many 

 of them only the barest trace of the other minerals. Of the accessory 

 minerals noted in the nodules feldspar is usually in largest amount. 



The ferromagnesian minerals, diallage and hornblende, in vary- 

 ing amounts are present in all the sections. In many of them, prob- 

 ably a majority, hornblende is subordinate in amount to diallage; 

 in others the two are in nearly equal proportion; and in still others 



