BUILDING STONES. 145 



Crystalline rocks.] 



little more than that in the red syenite, and for the same reason it has 

 ceased to be wrought at East St. Cloud. 



Microscopic characters of No. 1. The intimate structure of the fine-grained gray syenite from 

 East St. Cloud can be seen more minutely by examining the colored illustration, plate A, fig. 1, 

 which shows a magnified thin-section of the rock in ordinary light increased forty diameters. 

 The dark brown grain is biotite, the light brown and the brownish-green grains are hornblende, 

 and the green grains are chlorite. The most of the figure contains minerals that are not individ- 

 ualized in common light. They simply show a general cloudiness due to such included impurities 

 as ochreous limonite. The same field is shown in the same position, as it appears in polarized light 

 between crossed Nicols, in plate A, lig, 2, where that portion which was uuindividualized in com- 

 mon light is seen to consist of numerous different grains, the larger part of them being quartz. 



Microscopic characters' of No. 4. In plate A, figures 3 and 4, are seen the characters of this 

 rock magnified forty diameters, the former in ordinary light and the latter in polarized light 

 between crossed Nicols, the same field being represented in each. The green mineral is derived 

 from a change in the hornblende, and is as near chlorite as any established mineral ; but it shows 

 all stages of change from pure hornblende to a green, granular, confusedly polarizing substance. 

 When this green substance accumulates abundantly, it acquires a minutely foliated structure 

 which indicates chlorite. The large striated grain in the center (fig. 4) is plagioclase, and probably 

 albite. 



Microscopic characters of JVos. 6 ad!10. This rock has very evident grains of quartz, ortho- 

 clase and hornblende, but they are all more or less affected by included impurities. The quart/, 

 shows clouds and linear groups of bubbles and cavities, polarizing brilliantly. The orthoclase is 

 filled with impurities so as to be sometimes nearly opaque, and at other times has alternating but 

 nearly parallel, undulating ;md interrupted lines of light and light gray ; while the hornblende is 

 in about the same condition as described in the last. The figure (No. 5, plate A) represents a 

 microscopic field containing these three minerals magnified forty diameters, some of the horn- 

 blende evidently being darkened by emery derived from the polishing lap. With an objective 

 magnifying two or three hundred diameters these minerals, particularly the quartz and feldspar, 

 are seen to contain minute crystallites, some of them being acicular like apatite, or tremolite. 



Microscopic characters of No. 7. This rock is composed mainly of quartz and gray orthoclase, 

 which are about equally abundant. The quartz is pierced by numerous irregular fissures, and has 

 lines of pores, and in the pores are undetermined rnicrolites. The feldspar shows no twinning 

 striation, but has the generally clouded appearance of orthoclase when somewhat changed. 



At lake Suganaga are other granitic rocks. They extend over very large 

 areas, and are favorably exposed for quarrying. Some of them are quite 

 light-colored, or very similar to the "white granite" of Watab. They 

 change to a bedded light-colored syenitic gneiss. 



There is also a red syenite, which is seen back of Duluth in the hill- 

 ranges, and probably extends from there northwestwardly, in an interrupted 

 manner nearly to the international bo undary line. It is associated with gab- 

 bro (No. 2) intimately, and they interchange in areas so quickly that they 

 seem to have been once both molten at nearly the same time. This rock 

 has not attracted much attention, and has not been quarried except at 

 Beaver Bay, where Messrs. Wieland Brothers have used it in the filling of 

 the cribs of their dock. The analysis of this red syenite from Beaver Bay 



is given in the general table (No. 5) showing a content of silica amounting 

 10 



