LITHOLOGY. 1 6$ 



slates, and on fresh fractures it is red. This color results from the sepa- 

 ration of iron oxide. The rock is porphyritic ; but many crystals have 

 rotted away, and the stone is now full of cavities containing calcite. 



Mica Diorite. A diorite in which biotite replaces hornblende occurs 

 at Stewartstown. This is not a porphyritic rock, and, in addition to its 

 triclinic feldspar, which is probably anorthite, and its biotite, it contains 

 much calcite, and also some magnetite, pyrite, apatite, and chlorite. Some 

 such calcareous rocks were called hemithrene by Brongniart, but the 

 term is now obsolete; and the rocks so called are referred to diabase 

 and diorite, to which their nature and composition most closely relate 

 them. This specimen of diorite is rather different from all others col- 

 lected in the state, and whether it is an original product, or a result of 

 decomposition, is questionable. 



Gabbro. 



This rock in its mineral constituents is closely related to diabase, from 

 which our varieties are distinguished not only by the circumstance that 

 the pyroxene is of the foliated kind which is called diallage, but also by 

 their coarse granular structure, which in its details is much more like 

 that of granite than that of the diabase that has been described. 



Gabbro is found in immense masses in Waterville, and in the vicinity 

 of Mt. Washington. The relationships of its masses to the surrounding 

 strata are not so easily determined as are those of the little dykes of dia- 

 base and diorite, the walls of which are usually plainly seen ; but at some 

 points the rock possesses all the structure of an eruptive mass, and when 

 in other places this is not found, the evidence furnished by more favor- 

 able localities, as well as that furnished by allied rocks in other lands 

 where they have been more thoroughly investigated, must at present be 

 decisive. 



Our gabbros are coarse granular mixtures of labradorite, foliated augite 

 or diallage, olivine, and magnetic or titanic iron. Apatite and biotite are 

 the constant accessories. Hypersthene is sometimes prominent, and 

 sphene, chlorite, and pyrites are often present. The first four ingredi- 

 ents are macroscopically conspicuous, and the rest are identified in thin 

 sections. The prevailing color of the rock is dark gray, but it varies 



