GEOLOGICAL REPORT. 305 



dissolved into minute crystals. It contains many small crystals of a white feldspar, 

 also dark-brown mica, and less distinct, but very numerous throughout the matrix, 

 slender columns of dark green hornblende. 



No. 149. From the immediate neighborhood of No. 151. It is much less crystal- 

 line, more subcrystalline and uneven on the fracture. The matrix is grayish-green 

 (or rather a mixture of bright green, dark brown and white, the colors of the single 

 minerals), with many minute crystals of a greenish-white feldspar, and reddish-brown 

 columnar mica. The small crystals of the latter may, on superficial examination, be 

 readily mistaken for hvpersthene. No other minerals are crystallized out. 



No. 150. From the same locality; stands between the two preceding ones. 



No. 146. From the high conic mountain at the northern end of Round Prairie. 

 The weathered surface is reddish-brown. The gray matrix, granular, and composed 

 nearly altogether of microscopic crystals: it is thickly studded with mostly small crys- 

 tals of dark-brown mica and some quartz, which is more frequent in the matrix. No 

 hornblende is crystallized, at least not large enough to be recognized. 



No. 147. Near the locality of the former. It is the same rock more completely 

 crystallized. [t contains little matrix, and besides the feldspar and quartz, and the 

 lamellar hexagonal columns of brown mica, slender columns of greenish-black horn- 

 blende can well be distinguished. 



No. 148. From the same place. It has again much more dark matrix. The crys- 

 tals of feldspar are less numerous, but larger; the mica is dark-green, the matrix 

 quartzose, and hornblende could not be distinguished. 



No. 152. From the divide between Weber River and Silver Creek. It is a com- 

 pact, granular, dark-gray rock, more light-colored near the weathered surface. The 

 white feldspar and the hornblende are imperfectly crystallized. Small spots of oxide 

 of iron indicate that more hornblende, or probably mica, has -decayed. Other pieces 

 are a little better crystallized. 



No. 131. From near the same locality. It contain- only little whitish matrix, and 

 is mostly feldspar in tabular crystals, in its appearance much like some glassy feldspar 

 or sanidine, together with many columnar crystals of dark-green hornblende, mostly 

 thin, and a few laminae of brown mica. This specimen has quite the appearance of a 

 trachytic rock, but still I must consider it a diorite. 



No. 153. From Weber River, below Silver Creek. It has only very little gray 

 matrix between the coarse crystals of feldspar, the bright hexagonal lamina? of brown 

 mica, and the grains of quartz. This rock is nearly granitic. 



From the above we see that the minerals taking part in the composition of this 

 group of rocks are: a feldspar, dark brown mica, quartz, and dark green hornblende- 

 The latter was found only in well-crystallized specimens, and the want of one or the 

 other of these constituents in some of the rocks must be considered as local. It 

 seems, however, that the more the mica prevails and is well crystallized, the more does 

 the hornblende disappear and quartz come in. This is a rule which has frequently 

 been noticed with rocks of a much older group, the various syenites. We also see 

 how unsafe it is to base upon one specimen, perhaps indiscrimimnvh [ticked up, any 

 conclusion on the general composition of the igneous rocks of a district, by which we 

 might be enabled to recognize a contemporaneous formation at a distant point. 

 39bu 



