114 HENRY S. WASHINGTON 



The analyses of the two paisanites resemble each other 

 closely, the main differences being in silica, alumina, and fer- 

 rous oxide. These rocks are analogous to the grorudites of 

 Brogger, which, however, carry aegirite in place of soda-horn- 

 blende, and which are rather less acid than the Magnolia rock. 

 The replacement of alumina by ferric oxide in the grorudite is 

 to be noticed. The phenocrysts differ also in being alkali-feld- 

 spar, aegirite, and hornblende, while quartz occurs only in the 

 groundmass, and the color of the rock is dark green, rather than 

 blue, owing to the abundant aegirite. 



Solvsbergite. — The rocks belonging to this group are charac- 

 terized by the presence of alkali-feldspar with aegirite or a soda- 

 hornblende, with occasional biotite and very little or no quartz. 

 In Norway the structure is generally trachytic, which is not the 

 case in Essex county, but the similarity otherwise, both miner- 

 alogically and chemically, is so great that this structural differ- 

 ence may be overlooked. 



The Essex county solvsbergites are fine-grained and compact, 

 not very porphyritic, and of a gray or blue-gray color. As the 

 various dikes show rather diverse characters, they may be 

 described separately. 



One of these, from Dike 184, at Andrew's Point, Cape Ann, 

 has already been described 1 as composed essentially of feldspar 

 and glaucophane-riebeckite, with very little quartz. At that time, 

 however, only sections from the border of the dike were avail- 

 able. Since then I have studied sections from the center as well 

 with interesting results. The borders of the four-foot dike are 

 very fine-grained and compact and of blue-gray color. At the 

 center the rock becomes coarser, but is still fine-grained, and 

 is composed of small black specks in a light gray groundmass. 



Examining the sections under the microscope, it is seen that 

 in the borders the hornblende is nearly constantly glaucophane, 

 yet, as we approach the center, there are found streaks in which 

 a bright grass -green aegirite partly replaces it. At the center 

 the grain is larger, and the feldspars tend to become automor- 



1 H. S. Washington, Am. Jour. Sci. (4), VI, p. 176, 1898. 



