466 



Scientific Geology. 



Miner alogical Characters. 



1 Common Granite. This variety embraces nine tenths of the 

 granite in Massachusetts. All those compounds of quartz feldspar 

 and mica are included in it, which differ only in the size of the ingre- 

 dients, in the greater or less perfection of their crystalline structure, 

 and in their color. In these respects they do differ almost infinitely, 

 as may be seen by the numerous specimens from various parts of the 

 state in the collection. (Nos. 1372 to 1461.) Generally the quartz 

 is gray, sometimes smoky ; sometimes blue, and sometimes yellow. 

 The feldspar is ordinarily yellowish white ; sometimes green, as in 

 Southbridge ; sometimes blue, as in Leverett ; sometimes tinged with 

 purple, as in Palmer ; and more often flesh colored, as in the coarse 

 granite found in Blanford, Westfleld, Amherst, &c, and in the finer 

 grained granite in the southeast part of the state. The mica is more 

 commonly of a silver color ; sometimes of a straw or gold yellow, or 

 greenish ; sometimes of a brown color ; sometimes black ; sometimes 

 rose red ; and sometimes it is prismatic, as in Russell and Norwich. 



In magnitude the ingredients vary from that of masses one and 

 even two feet in diameter, to those so small that they can be distin- 

 guished but with difficulty by the naked eye. Those granites that 

 possess a fine grain are the only varieties that are employed for archi- 

 tectural purposes. The coarsest varieties are generally found in 

 veins. 



2. Pseudomorphous Granite. This is a variety that exhibits a 

 structure so peculiar, that I have thought it deserves a distinct notice. 

 Suppose the quartz and feldspar requisite to form a coarse granite to be 

 united into a solid mass. Suppose the mass to be now penetrated in va- 

 rious directions by the blade of a thin knife, and the cavities thus pro- 

 duced to be filled by plates of mica not more than one fiftieth of an inch 

 thick. Although these plates would form solid angles, they would not 

 intersect one another; and so it is in the rock. The smallest fragments 

 of the quartz and feldspar are often separated by the mica ; but I have 

 never seen one plate of that mineral intersect another. The solid an- 

 gles which these plates form in the quartz and feldspar, however, ap- 

 pear like the projecting angles of crystals, and hence I have applied to 

 this granite the term pseudomorphous. The mica is usually of a deep 

 bronze colour and often the plates are four or five inches across. 

 (No. 1462.) 



3. Porphyritic Granite. (Nos. 1463 to 1470.) In this variety, 

 besides the ingredients composing the mass of the rock, and which 

 are quartz, feldspar and mica, distinct imbedded crystals of feldspar 



