384 



Scientific Geology. 



tion of parts. But it is difficult to describe the exact nature of that 

 peculiarity. 



2. Schistose Gneiss. (Nos. 993 to 1022.) This is probably the most 

 common variety of our gneiss. The structure is foliated like that of 

 mica slate : though sometimes granular with a laminar tendency. Some 

 of the best quarries in the state I regard as belonging to this variety. 

 It passes frequently into mica slate by the disappearance of the feld- 

 spar. 



3. Laminar Gneiss. In this variety the different ingredients occupy 

 distinct layers. When the mica is black, or there is an interlamination 

 of hornblende, the different laminae are remarkably distinct and reg- 

 ular. In some instances, perhaps, this rock may be regarded as 

 composed of alternating layers of gneiss and mica slate, or horn- 

 blende slate. (Nos. 1023 to 1040.) 



4. Porphyritic Gneiss. The structure of this variety is more or 

 less slaty. But it embraces distinct crystalline masses of foliated 

 feldspar. Most commonly these masses are somewhat ovoid; but 

 in some instances they present the regular forms of the crystals. 

 The color is sometimes white or gray : but a reddish hue predomi- 

 nates. The imbedded masses vary in size from a quarter of an inch 

 in their longest direction, to two inches ; and they sometimes consti- 

 tute the largest portion of the rock. This variety sometimes answers 

 well for architectural purposes. (Nos. 1041 to 1050.) 



5. Amphibolic Gneiss. I thus denominate that variety which takes 

 a small proportion of hornblende into its composition: not sufficient 

 to form hornblende slate. This mineral is usually disseminated in 

 black foliated masses, from the size of a pin's head to half an inch in 

 diameter, through the rock. It occurs only in the vicinity of horn- 

 blende slate. (Nos. 1051 to 1054.) 



In the southeastern part of Worcester county is a beautiful rock, 

 extensively quarried, which I refer to this variety. One meets with 

 it abundantly in Mendon, Grafton, and the south part of Worcester, 

 in blocks got out for building ; but I do not know where are its quar- 

 ries. The rock appears to be a granitic gneiss, composed almost en- 

 tirely of quartz and feldspar, through which are disseminated numer- 

 ous black crystalline masses of hornblende, which have a somewhat 

 parallel arrangement. This parallelism is almost the only mark by 

 which I distinguish this rock from granite. It might with propriety 

 be termed sienitic gneiss. (Nos. 983 and 986.) 



6. Epidotic Gneiss. This variety usually contains hornblende as 



