30 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



rate layers and the granulation of much of the quartz, especially 

 that surrounding the microcline phenocrysts and some crushing 

 of the microcline itself. 



Still another vastly different type of dike rock occurs. That is 

 a biotite hornblendite composed in large part of those two minerals 

 together with some apatite, pyrrhotite and magnetite, a little color- 

 less amphibole and, in one instance, monoclinic pyroxene, in 

 another, a little andesine and quartz. In plane polarized light the 

 hornblende is light green but shows strong pleochroism — blue- 

 green parallel with Z and light yellow-green parallel with X with a 

 maximum extinction angle of twenty-nine degrees. It frequently 

 merges into a colorless, non-pleochroic am.phibole with a slightly 

 lower extinction angle and birefringence. This is not a case of 

 sharp zoning. The two usually blend irregularly with green at 

 the center or are patchily distributed one within the other. The 

 texture of this rock is massive and the biotite is commonly devel- 

 oped along the prismatic cleavage planes of the hornblende. 

 (Plate VII b.) 



Three small dikes of this type occur near the eastern edge of 

 the diorite but the great majority are grouped around the central 

 section of the tunnel just east and west of the Bantam River 

 crossing. They all strike north to northeast and are between ten 

 and two hundred feet thick. 



There is no identical rock that outcrops in this or in neighboring 

 regions. The norites, gabbros, and related rocks of Mount Pros- 

 pect^ approach it most closely. They are unmetamorphosed intru- 

 sives younger than the main mass of the diorite and though they 

 usually contain considerable monoclinic or orthorhombic pyroxene, 

 some are nearly all hornblende and mica. 



There is some similarity, then, between the two groups, espe- 

 cially since the biotite hornblendite dike in the tunnel nearest to 

 Mount Prospect contains a good deal of monoclinic pyroxene, and 

 pyrrhotite is a constant accessory in the dikes as well as being the 

 cause of the long-abandoned nickel prospects on Mount Prospect 

 itself. 



The complexity of types grouped here under the term Brookfield 

 diorite is considerable and does not represent a simple sequence of 

 intrusions. A hand specimen three by three inches Avill often 

 show both coarse and fine-grained types with a good deal of dif- 

 ference in the amount of feldspar present, and the coarse-grained 

 diorite grades quickly but evenly into the fine-grained, granular 

 type with small but distinct plagioclase phenocrysts. A certain 

 amount of segregation or differentiation in place has occurred but 

 that cannot explain all of the variations. The granodiorite and 

 the microcline porphyry types are distinct dikes and, as was pointed 

 out above, much of the diorite is rather strongly foliated. Those 



1 On Two New Occurrences of the "Cortland Series" of Rocks within the State of 

 Connecticut. W. H. Hobbs, Fest. H. Rosenbusch, pp. 25-48, Stuttgart, 1906. 



